Remove ServiceRegistry

We don't need this anymore now that we have Mozart. The engine recently
stopped support it also.

packages/flutter/lib/services.dart

@@ -20,5 +20,3 @@ export 'src/services/image_decoder.dart';
 export 'src/services/image_resource.dart';
 export 'src/services/keyboard.dart';
 export 'src/services/print.dart';
-export 'src/services/service_registry.dart';
-

packages/flutter/lib/src/services/service_registry.dart

-// Copyright 2015 The Chromium 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 'dart:ui' as ui;
-
-import 'package:mojo_services/mojo/service_registry.mojom.dart';
-import 'package:mojo/core.dart' as core;
-
-ServiceRegistryProxy _initServiceRegistryProxy() {
-  core.MojoHandle serviceRegistryHandle = new core.MojoHandle(ui.takeServiceRegistry());
-  if (!serviceRegistryHandle.isValid)
-    return null;
-  return new ServiceRegistryProxy.fromHandle(serviceRegistryHandle);
-}
-
-final ServiceRegistryProxy _serviceRegistryProxy = _initServiceRegistryProxy();
-final ServiceRegistry serviceRegistry = _serviceRegistryProxy?.ptr;