pkg/unittest: remove deprecated features

pkg/unittest/lib/mirror_matchers.dart

Index: pkg/unittest/lib/mirror_matchers.dart
diff --git a/pkg/unittest/lib/mirror_matchers.dart b/pkg/unittest/lib/mirror_matchers.dart
deleted file mode 100644
index d144554e0339331ac381bc7b2f925a2258907ed3..0000000000000000000000000000000000000000
--- a/pkg/unittest/lib/mirror_matchers.dart
+++ /dev/null
@@ -1,12 +0,0 @@
-// Copyright (c) 2013, 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.mirror_matchers` has been moved to the `matcher` package.
-///
-/// Add `matcher` to your `pubspec.yaml` file and import it via
-/// `import 'package:matcher/mirror_matchers.dart';`
-@deprecated
-library unittest.mirror_matchers;
-
-export 'package:matcher/mirror_matchers.dart';