Remove the old type system for proxied resources. These were no longer being used.

ppapi/proxy/mock_resource.h

 // Copyright (c) 2011 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 PPAPI_PROXY_MOCK_RESOURCE_H_
 #define PPAPI_PROXY_MOCK_RESOURCE_H_
 
 #include "ppapi/c/pp_instance.h"
 #include "ppapi/proxy/plugin_resource.h"
 
 namespace pp {
 namespace proxy {
 
 class MockResource : public PluginResource {
  public:
   MockResource(const HostResource& resource);
   virtual ~MockResource();
 
-  // Resource overrides.
-  virtual MockResource* AsMockResource();
-
  private:
   DISALLOW_COPY_AND_ASSIGN(MockResource);
 };
 
 }  // namespace proxy
 }  // namespace pp
 
 #endif  // PPAPI_PROXY_MOCK_RESOURCE_H_