Remove dead ContextMenu code from Blink core

Source/core/loader/EmptyClients.cpp

 /*
  * Copyright (C) 2006 Eric Seidel <eric@webkit.org>
  * Copyright (C) 2008, 2009, 2012 Apple Inc. All rights reserved.
  * Copyright (C) Research In Motion Limited 2011. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
@@ skipping 129 matching lines @@
 }
 
 void EmptyEditorClient::registerUndoStep(PassRefPtr<UndoStep>)
 {
 }
 
 void EmptyEditorClient::registerRedoStep(PassRefPtr<UndoStep>)
 {
 }
 
-PassOwnPtr<ContextMenu> EmptyContextMenuClient::customizeMenu(PassOwnPtr<ContextMenu>)
-{
-    return nullptr;
-}
-
 void EmptyFrameLoaderClient::didRequestAutocomplete(PassRefPtr<FormState>)
 {
 }
 
 }