remove mock doublon

flutter/test/load_contexts_integrations_test.dart

@@ -1,13 +1,10 @@
 import 'package:flutter/services.dart';
 import 'package:flutter_test/flutter_test.dart';
-import 'package:mockito/mockito.dart';
 import 'package:sentry/sentry.dart';
 import 'package:sentry_flutter/sentry_flutter.dart';
 
 import 'mocks.dart';
 
-class MockTransport extends Mock implements Transport {}
-
 void main() {
   const MethodChannel _channel = MethodChannel('sentry_flutter');