pkg/unittest: remove deprecated features

pkg/unittest/lib/mock.dart

Index: pkg/unittest/lib/mock.dart
diff --git a/pkg/unittest/lib/mock.dart b/pkg/unittest/lib/mock.dart
deleted file mode 100644
index 18b6537cf2188f630517a8e37ff170650817ffd2..0000000000000000000000000000000000000000
--- a/pkg/unittest/lib/mock.dart
+++ /dev/null
@@ -1,12 +0,0 @@
-// Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
-// for details. All rights reserved. Use of this source code is governed by a
-// BSD-style license that can be found in the LICENSE file.
-
-/// `unittest.mock` has been moved to the `mock` package.
-///
-/// Add `mock` to your `pubspec.yaml` file and import it via
-/// `import 'package:mock/mock.dart';`
-@deprecated
-library unittest.mock;
-
-export 'package:mock/mock.dart';