fix: remove unused mock

test/how_to_play/how_to_play_dialog_test.dart

@@ -3,13 +3,10 @@ import 'package:flutter_test/flutter_test.dart';
 import 'package:mocktail/mocktail.dart';
 import 'package:pinball/how_to_play/how_to_play.dart';
 import 'package:pinball/l10n/l10n.dart';
-import 'package:pinball_audio/pinball_audio.dart';
 import 'package:platform_helper/platform_helper.dart';
 
 import '../helpers/helpers.dart';
 
-class _MockPinballAudio extends Mock implements PinballAudio {}
-
 class _MockPlatformHelper extends Mock implements PlatformHelper {}
 
 void main() {