Delete `flutter_gallery_mac__start_up`. (#111147)

dev/devicelab/bin/tasks/flutter_gallery_mac__start_up.dart

-// Copyright 2014 The Flutter Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style license that can be
-// found in the LICENSE file.
-
-import 'package:flutter_devicelab/framework/framework.dart';
-import 'package:flutter_devicelab/tasks/perf_tests.dart';
-
-Future<void> main() async {
-  // It's intended to use the Gallery startup test as a smoke test on macOS
-  // Catalina.
-  await task(createFlutterGalleryStartupTest());
-}