[macOS] Remove FlutterApplication class (#40939)

The need for the FlutterApplication NSApplication subclass was
eliminated in 8679b67. The relevant app
termination logic was moved into FlutterAppDelegate in that patch.
Removing FlutterApplication needs to be done in three stages:
1. Migrate logic to FlutterAppDelegate (done in above patch)
2. Update Flutter tool template. Update migrator to migrate all apps
using FlutterApplication back to NSApplication. This is done in
flutter/flutter#124173.
3. Eliminate FlutterApplication.h header since all references to it have
now been removed.

No tests since, as described above, this is the removal of unused (as of
step 2 above) code.

Issue: flutter/flutter#30735

## Pre-launch Checklist

- [X] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [X] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [X] I read and followed the [Flutter Style Guide] and the [C++,
Objective-C, Java style guides].
- [X] I listed at least one issue that this PR fixes in the description
above.
- [ ] I added new tests to check the change I am making or feature I am
adding, or Hixie said the PR is test-exempt. See [testing the engine]
for instructions on writing and running engine tests.
- [X] I updated/added relevant documentation (doc comments with `///`).
- [X] I signed the [CLA].
- [X] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel
on [Discord].

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Author: cbracken

ci/licenses_golden/licenses_flutter

@@ -2603,7 +2603,6 @@ ORIGIN: ../../../flutter/shell/platform/darwin/ios/platform_view_ios.mm + ../../
 ORIGIN: ../../../flutter/shell/platform/darwin/ios/rendering_api_selection.h + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/darwin/ios/rendering_api_selection.mm + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/darwin/macos/framework/Headers/FlutterAppDelegate.h + ../../../flutter/LICENSE
-ORIGIN: ../../../flutter/shell/platform/darwin/macos/framework/Headers/FlutterApplication.h + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/darwin/macos/framework/Headers/FlutterDartProject.h + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/darwin/macos/framework/Headers/FlutterEngine.h + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/darwin/macos/framework/Headers/FlutterMacOS.h + ../../../flutter/LICENSE
@@ -2616,7 +2615,6 @@ ORIGIN: ../../../flutter/shell/platform/darwin/macos/framework/Source/Accessibil
 ORIGIN: ../../../flutter/shell/platform/darwin/macos/framework/Source/AccessibilityBridgeMacTest.mm + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterAppDelegate.mm + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterAppDelegate_Internal.h + ../../../flutter/LICENSE
-ORIGIN: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterApplication.mm + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterBackingStore.h + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterBackingStore.mm + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterChannelKeyResponder.h + ../../../flutter/LICENSE
@@ -5196,7 +5194,6 @@ FILE: ../../../flutter/shell/platform/darwin/ios/platform_view_ios.mm
 FILE: ../../../flutter/shell/platform/darwin/ios/rendering_api_selection.h
 FILE: ../../../flutter/shell/platform/darwin/ios/rendering_api_selection.mm
 FILE: ../../../flutter/shell/platform/darwin/macos/framework/Headers/FlutterAppDelegate.h
-FILE: ../../../flutter/shell/platform/darwin/macos/framework/Headers/FlutterApplication.h
 FILE: ../../../flutter/shell/platform/darwin/macos/framework/Headers/FlutterDartProject.h
 FILE: ../../../flutter/shell/platform/darwin/macos/framework/Headers/FlutterEngine.h
 FILE: ../../../flutter/shell/platform/darwin/macos/framework/Headers/FlutterMacOS.h
@@ -5210,7 +5207,6 @@ FILE: ../../../flutter/shell/platform/darwin/macos/framework/Source/Accessibilit
 FILE: ../../../flutter/shell/platform/darwin/macos/framework/Source/AccessibilityBridgeMacTest.mm
 FILE: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterAppDelegate.mm
 FILE: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterAppDelegate_Internal.h
-FILE: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterApplication.mm
 FILE: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterBackingStore.h
 FILE: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterBackingStore.mm
 FILE: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterChannelKeyResponder.h

shell/platform/darwin/macos/BUILD.gn

@@ -38,7 +38,6 @@ _framework_binary_subpath = "Versions/A/$_flutter_framework_name"
 # the Flutter engine source root.
 _flutter_framework_headers = [
   "framework/Headers/FlutterAppDelegate.h",
-  "framework/Headers/FlutterApplication.h",
   "framework/Headers/FlutterDartProject.h",
   "framework/Headers/FlutterEngine.h",
   "framework/Headers/FlutterMacOS.h",
@@ -58,7 +57,6 @@ source_set("flutter_framework_source") {
     "framework/Source/AccessibilityBridgeMac.h",
     "framework/Source/AccessibilityBridgeMac.mm",
     "framework/Source/FlutterAppDelegate.mm",
-    "framework/Source/FlutterApplication.mm",
     "framework/Source/FlutterBackingStore.h",
     "framework/Source/FlutterBackingStore.mm",
     "framework/Source/FlutterChannelKeyResponder.h",

shell/platform/darwin/macos/framework/Headers/FlutterApplication.h

@@ -3,7 +3,6 @@
 // found in the LICENSE file.
 
 #import "FlutterAppDelegate.h"
-#import "FlutterApplication.h"
 #import "FlutterBinaryMessenger.h"
 #import "FlutterChannels.h"
 #import "FlutterCodecs.h"