Revert of Add TabContainer to Showcase

ios/showcase/test/showcase_matchers.mm

Index: ios/showcase/test/showcase_matchers.mm
diff --git a/ios/showcase/test/showcase_matchers.mm b/ios/showcase/test/showcase_matchers.mm
deleted file mode 100644
index 9eb945f2e7b96c27c068221498d206bed0e89036..0000000000000000000000000000000000000000
--- a/ios/showcase/test/showcase_matchers.mm
+++ /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.
-
-#import "ios/showcase/test/showcase_matchers.h"
-
-#if !defined(__has_feature) || !__has_feature(objc_arc)
-#error "This file requires ARC support."
-#endif
-
-namespace showcase_matchers {
-
-id<GREYMatcher> FirstLevelBackButton() {
-  return grey_kindOfClass(
-      NSClassFromString(@"_UINavigationBarBackIndicatorView"));
-}
-
-}  // namespace showcase_matchers