Allow all tests to run with --update-goldens. (#31562)

Previously benchmark_test.dart would break if run with --update-goldens.
Now the entire test suite can be run with --update-goldens.

packages/flutter_test/lib/src/binding.dart

@@ -1106,12 +1106,6 @@ enum LiveTestWidgetsFlutterBindingFramePolicy {
 /// doesn't trigger a paint, since then you could not see anything
 /// anyway.)
 class LiveTestWidgetsFlutterBinding extends TestWidgetsFlutterBinding {
-  @override
-  void initInstances() {
-    super.initInstances();
-    assert(!autoUpdateGoldenFiles);
-  }
-
   @override
   bool get inTest => _inTest;
   bool _inTest = false;

packages/flutter_test/lib/src/goldens.dart

@@ -124,7 +124,6 @@ class TrivialComparator implements GoldenFileComparator {
 
   @override
   Future<void> update(Uri golden, Uint8List imageBytes) {
-    // [autoUpdateGoldenFiles] should never be set in a live widget binding.
     throw StateError('goldenFileComparator has not been initialized');
   }
 }