fix: remove unused import

Author: holzgeist

lib/src/media_stream_track_impl.dart

@@ -1,7 +1,6 @@
 import 'dart:async';
 import 'dart:js_interop';
 import 'dart:js_interop_unsafe';
-import 'dart:js_util' as js;
 import 'dart:typed_data';
 
 import 'package:web/web.dart' as web;