Remove unnecessary RadioGroup migration TODOs (#180105)

Part of: https://github.com/flutter/flutter/issues/179088

Previous:
* https://github.com/flutter/flutter/pull/179158
* https://github.com/flutter/flutter/pull/179091

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above.
- [x] I updated/added relevant documentation (doc comments with `///`).
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examples/api/test/material/menu_anchor/radio_menu_button.0_test.dart

@@ -64,9 +64,6 @@ void main() {
           .widget<RadioMenuButton<Color>>(
             find.byType(RadioMenuButton<Color>).at(0),
           )
-          // TODO(loic-sharma): Migrate to RadioGroup.
-          // https://github.com/flutter/flutter/issues/179088
-          // ignore: deprecated_member_use
           .groupValue,
       equals(Colors.green),
     );
@@ -86,9 +83,6 @@ void main() {
           .widget<RadioMenuButton<Color>>(
             find.byType(RadioMenuButton<Color>).at(0),
           )
-          // TODO(loic-sharma): Migrate to RadioGroup.
-          // https://github.com/flutter/flutter/issues/179088
-          // ignore: deprecated_member_use
           .groupValue,
       equals(Colors.red),
     );
@@ -108,9 +102,6 @@ void main() {
           .widget<RadioMenuButton<Color>>(
             find.byType(RadioMenuButton<Color>).at(0),
           )
-          // TODO(loic-sharma): Migrate to RadioGroup.
-          // https://github.com/flutter/flutter/issues/179088
-          // ignore: deprecated_member_use
           .groupValue,
       equals(Colors.blue),
     );

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

@@ -8352,8 +8352,6 @@ typedef StatefulWidgetBuilder = Widget Function(BuildContext context, StateSette
 /// This example shows using an inline StatefulBuilder that rebuilds and that
 /// also has state.
 ///
-// TODO(loic-sharma): Migrate to RadioGroup.
-// https://github.com/flutter/flutter/issues/179088
 /// ```dart
 /// await showDialog<void>(
 ///   context: context,