Remove more dead app list code.

ui/app_list/app_list_folder_item.h

 // Copyright 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_APP_LIST_FOLDER_ITEM_H_
 #define UI_APP_LIST_APP_LIST_FOLDER_ITEM_H_
 
 #include <stddef.h>
 
 #include <string>
@@ skipping 51 matching lines @@
   const FolderImage& folder_image() const { return folder_image_; }
 
   FolderType folder_type() const { return folder_type_; }
 
   // AppListItem overrides:
   void Activate(int event_flags) override;
   const char* GetItemType() const override;
   ui::MenuModel* GetContextMenuModel() override;
   AppListItem* FindChildItem(const std::string& id) override;
   size_t ChildItemCount() const override;
-  void OnExtensionPreferenceChanged() override;
   bool CompareForTest(const AppListItem* other) const override;
 
   // Returns an id for a new folder.
   static std::string GenerateId();
 
   // FolderImageObserver overrides:
   void OnFolderImageUpdated() override;
 
  private:
   // The type of folder; may affect behavior of folder views.
   const FolderType folder_type_;
 
   // List of items in the folder.
   std::unique_ptr<AppListItemList> item_list_;
 
   FolderImage folder_image_;
 
   DISALLOW_COPY_AND_ASSIGN(AppListFolderItem);
 };
 
 }  // namespace app_list
 
 #endif  // UI_APP_LIST_APP_LIST_FOLDER_ITEM_H_