Revert of Add TabContainer to Showcase

ios/showcase/test/showcase_matchers.h

Index: ios/showcase/test/showcase_matchers.h
diff --git a/ios/showcase/test/showcase_matchers.h b/ios/showcase/test/showcase_matchers.h
deleted file mode 100644
index e8fb5ceb53a3eccd239029ae323f559b477cb729..0000000000000000000000000000000000000000
--- a/ios/showcase/test/showcase_matchers.h
+++ /dev/null
@@ -1,18 +0,0 @@
-// Copyright 2017 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.
-
-#ifndef IOS_SHOWCASE_TEST_SHOWCASE_MATCHERS_H_
-#define IOS_SHOWCASE_TEST_SHOWCASE_MATCHERS_H_
-
-#import <EarlGrey/EarlGrey.h>
-
-namespace showcase_matchers {
-
-// Matcher for the back button on screens presented from the Showcase home
-// screen.
-id<GREYMatcher> FirstLevelBackButton();
-
-}  // namespace showcase_matchers
-
-#endif  // IOS_SHOWCASE_TEST_SHOWCASE_MATCHERS_H_