Remove unused import from test

Author: mvanbeusekom

packages/webview_flutter/webview_flutter_android/test/webview_android_widget_test.dart

@@ -3,7 +3,6 @@
 // found in the LICENSE file.
 
 import 'dart:async';
-import 'dart:convert';
 
 import 'package:flutter/widgets.dart';
 import 'package:flutter_test/flutter_test.dart';