Remove early Flutter GPU prototype from ui/experiments. (flutter#51367)

We moved to lib/gpu some time ago!
(Initially added with flutter#42228)

Author: bdero

ci/licenses_golden/licenses_flutter

@@ -40251,7 +40251,6 @@ ORIGIN: ../../../flutter/lib/ui/dart_runtime_hooks.h + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/lib/ui/dart_ui.cc + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/lib/ui/dart_ui.h + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/lib/ui/dart_wrapper.h + ../../../flutter/LICENSE
-ORIGIN: ../../../flutter/lib/ui/experiments/gpu.dart + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/lib/ui/experiments/scene.dart + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/lib/ui/experiments/setup_hooks.dart + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/lib/ui/experiments/ui.dart + ../../../flutter/LICENSE
@@ -43108,7 +43107,6 @@ FILE: ../../../flutter/lib/ui/dart_runtime_hooks.h
 FILE: ../../../flutter/lib/ui/dart_ui.cc
 FILE: ../../../flutter/lib/ui/dart_ui.h
 FILE: ../../../flutter/lib/ui/dart_wrapper.h
-FILE: ../../../flutter/lib/ui/experiments/gpu.dart
 FILE: ../../../flutter/lib/ui/experiments/scene.dart
 FILE: ../../../flutter/lib/ui/experiments/setup_hooks.dart
 FILE: ../../../flutter/lib/ui/experiments/ui.dart

lib/ui/experiments/gpu.dart

@@ -49,5 +49,4 @@ part 'setup_hooks.dart';
 part '../text.dart';
 part '../window.dart';
 
-part 'gpu.dart';
 part 'scene.dart';