Revert "iOS: Migrate FlutterEngine to ARC (#55590)" (#55699)

Causing flakes on the _Mac mac_unopt (Cocoon)_ shard.

This reverts commit 449df25.

Issue: flutter/flutter#156177

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style

Author: cbracken

shell/platform/darwin/ios/BUILD.gn

@@ -70,9 +70,7 @@ source_set("flutter_framework_source_arc") {
     "framework/Source/FlutterDartVMServicePublisher.mm",
     "framework/Source/FlutterEmbedderKeyResponder.h",
     "framework/Source/FlutterEmbedderKeyResponder.mm",
-    "framework/Source/FlutterEngine.mm",
     "framework/Source/FlutterEngineGroup.mm",
-    "framework/Source/FlutterEngine_Internal.h",
     "framework/Source/FlutterHeadlessDartRunner.mm",
     "framework/Source/FlutterKeyPrimaryResponder.h",
     "framework/Source/FlutterKeySecondaryResponder.h",
@@ -188,6 +186,8 @@ source_set("flutter_framework_source") {
     # iOS embedder is migrating to ARC.
     # New files are highly encouraged to be in ARC.
     # To add new files in ARC, add them to the `flutter_framework_source_arc` target.
+    "framework/Source/FlutterEngine.mm",
+    "framework/Source/FlutterEngine_Internal.h",
     "framework/Source/FlutterViewController.mm",
     "framework/Source/FlutterViewController_Internal.h",
     "platform_view_ios.h",