remove Unused import.

Author: cloudwebrtc

lib/src/rtc_data_channel_impl.dart

@@ -1,6 +1,5 @@
 import 'dart:async';
 import 'dart:js_interop';
-import 'dart:js_util' as jsutil;
 import 'package:web/web.dart' as web;
 import 'package:webrtc_interface/webrtc_interface.dart';