<extensionoptions>: Remove dead code

extensions/common/api/extension_options_internal.idl

 // Copyright 2014 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.
 
 // Internal API for the <extensiontoptions> tag
 namespace extensionOptionsInternal {
   dictionary SizeChangedOptions {
     long oldWidth;
     long oldHeight;
     long newWidth;
     long newHeight;
   };
 
   dictionary PreferredSizeChangedOptions {
     double width;
     double height;
   };
 
   interface Events {
     static void onClose();
     static void onLoad();
-    static void onSizeChanged(SizeChangedOptions options);
     static void onPreferredSizeChanged(PreferredSizeChangedOptions options);
   };
 };