Revert "Fixed AnimatedSwitcher chain produced duplicates (#106962)" (#107318)

This reverts commit 0d3cc92d.

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

@@ -222,7 +222,6 @@ class AnimatedSwitcher extends StatefulWidget {
   /// This is an [AnimatedSwitcherTransitionBuilder] function.
   static Widget defaultTransitionBuilder(Widget child, Animation<double> animation) {
     return FadeTransition(
-      key: ValueKey<Key?>(child.key),
       opacity: animation,
       child: child,
     );
@@ -395,6 +394,6 @@ class _AnimatedSwitcherState extends State<AnimatedSwitcher> with TickerProvider
   @override
   Widget build(BuildContext context) {
     _rebuildOutgoingWidgetsIfNeeded();
-    return widget.layoutBuilder(_currentEntry?.transition, _outgoingWidgets!.where((Widget outgoing) => outgoing.key != _currentEntry?.transition.key).toSet().toList());
+    return widget.layoutBuilder(_currentEntry?.transition, _outgoingWidgets!);
   }
 }

packages/flutter/test/widgets/animated_switcher_test.dart

@@ -415,25 +415,6 @@ void main() {
       );
     }
   });
-
-  testWidgets('AnimatedSwitcher does not duplicate animations if the same child is entered twice.', (WidgetTester tester) async {
-    Future<void> pumpChild(Widget child) async {
-      return tester.pumpWidget(
-        Directionality(
-          textDirection: TextDirection.ltr,
-          child: AnimatedSwitcher(
-            duration: const Duration(milliseconds: 1000),
-            child: child,
-          ),
-        ),
-      );
-    }
-    await pumpChild(const Text('1', key: Key('1')));
-    await pumpChild(const Text('2', key: Key('2')));
-    await pumpChild(const Text('1', key: Key('1')));
-    await tester.pump(const Duration(milliseconds: 1000));
-    expect(find.text('1'), findsOneWidget);
-  });
 }
 
 class StatefulTest extends StatefulWidget {