Clean up forward declarations in Source/core

Source/core/dom/custom/CustomElementDescriptor.h

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 73 matching lines @@
 private:
     AtomicString m_type;
     AtomicString m_namespaceURI;
     AtomicString m_localName;
 };
 
 } // namespace blink
 
 namespace WTF {
 
-template<typename T> struct DefaultHash;
 template<> struct DefaultHash<blink::CustomElementDescriptor> {
     typedef blink::CustomElementDescriptorHash Hash;
 };
 
 } // namespace WTF
 
 #endif // CustomElementDescriptor_h