Remove old (dead) app list code.

ui/app_list/views/folder_header_view.h

 // Copyright (c) 2013 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 UI_APP_LIST_VIEWS_FOLDER_HEADER_VIEW_H_
 #define UI_APP_LIST_VIEWS_FOLDER_HEADER_VIEW_H_
 
 #include <string>
 
 #include "base/macros.h"
@@ skipping 62 matching lines @@
                        const base::string16& new_contents) override;
 
   // views::ButtonListener overrides:
   void ButtonPressed(views::Button* sender, const ui::Event& event) override;
 
   // AppListItemObserver overrides:
   void ItemNameChanged() override;
 
   AppListFolderItem* folder_item_;  // Not owned.
 
-  views::ImageButton* back_button_;  // Owned by views hierarchy.
   FolderNameView* folder_name_view_;  // Owned by views hierarchy.
 
   const base::string16 folder_name_placeholder_text_;
 
   FolderHeaderViewDelegate* delegate_;
 
   bool folder_name_visible_;
 
   DISALLOW_COPY_AND_ASSIGN(FolderHeaderView);
 };
 
 }  // namespace app_list
 
 #endif  // UI_APP_LIST_VIEWS_FOLDER_HEADER_VIEW_H_