Remove deprecated TextSelectionOverlay.fadeDuration (#134485)

Part of https://github.com/flutter/flutter/issues/133171

This was deprecated in https://github.com/flutter/flutter/pull/100381
Th replacement is to use `SelectionOverlay.fadeDuration` instead.
This migration is supported by dart fix. âœ

packages/flutter/lib/src/widgets/text_selection.dart

@@ -374,13 +374,6 @@ class TextSelectionOverlay {
   /// {@endtemplate}
   final BuildContext context;
 
-  /// Controls the fade-in and fade-out animations for the toolbar and handles.
-  @Deprecated(
-    'Use `SelectionOverlay.fadeDuration` instead. '
-    'This feature was deprecated after v2.12.0-4.1.pre.'
-  )
-  static const Duration fadeDuration = SelectionOverlay.fadeDuration;
-
   // TODO(mpcomplete): what if the renderObject is removed or replaced, or
   // moves? Not sure what cases I need to handle, or how to handle them.
   /// The editable line in which the selected text is being displayed.

packages/flutter/test/widgets/text_selection_test.dart

@@ -99,10 +99,6 @@ void main() {
     );
   }
 
-  test('TextSelectionOverlay.fadeDuration exist', () async {
-    expect(TextSelectionOverlay.fadeDuration, SelectionOverlay.fadeDuration);
-  });
-
   testWidgets('a series of taps all call onTaps', (WidgetTester tester) async {
     await pumpGestureDetector(tester);
     await tester.tapAt(const Offset(200, 200));