fix: Ambiguation is not needed in render box anymore (#2711)

Since we are requiring a SDK higher than when the instance could be null we don't need to do the ambiguate call anymore.

packages/flame/lib/src/game/game_render_box.dart

@@ -131,25 +131,15 @@ class GameRenderBox extends RenderBox with WidgetsBindingObserver {
   }
 
   void _bindLifecycleListener() {
-    _ambiguate(WidgetsBinding.instance)!.addObserver(this);
+    WidgetsBinding.instance.addObserver(this);
   }
 
   void _unbindLifecycleListener() {
-    _ambiguate(WidgetsBinding.instance)!.removeObserver(this);
+    WidgetsBinding.instance.removeObserver(this);
   }
 
   @override
   void didChangeAppLifecycleState(AppLifecycleState state) {
     game.lifecycleStateChange(state);
   }
 }
-
-/// This allows a value of type T or T?
-/// to be treated as a value of type T?.
-///
-/// We use this so that APIs that have become
-/// non-nullable can still be used with `!` and `?`
-/// to support older versions of the API as well.
-///
-/// See more: https://docs.flutter.dev/development/tools/sdk/release-notes/release-notes-3.0.0
-T? _ambiguate<T>(T? value) => value;