Remove unused local variable

Author: creativecreatorormaybenot

packages/video_player/video_player_platform_interface/lib/messages.dart

@@ -429,8 +429,6 @@ abstract class TestHostVideoPlayerApi {
           'dev.flutter.pigeon.VideoPlayerApi.initialize',
           StandardMessageCodec());
       channel.setMockMessageHandler((dynamic message) async {
-        final Map<dynamic, dynamic> mapMessage =
-            message as Map<dynamic, dynamic>;
         api.initialize();
         return <dynamic, dynamic>{};
       });