Remove some dead code in SVG "mixins"

third_party/WebKit/Source/core/svg/SVGURIReference.cpp

 /*
  * Copyright (C) 2004, 2005, 2008 Nikolas Zimmermann <zimmermann@kde.org>
  * Copyright (C) 2004, 2005, 2006 Rob Buis <buis@kde.org>
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
  * This library is distributed in the hope that it will be useful,
@@ skipping 87 matching lines @@
         return externalDocument->getElementById(id);
     }
 
     // Exit early if the referenced url is external, and we have no externalDocument given.
     if (isExternalURIReference(iri, document))
         return nullptr;
 
     return treeScope.getElementById(id);
 }
 
-void SVGURIReference::addSupportedAttributes(HashSet<QualifiedName>& supportedAttributes)
-{
-    supportedAttributes.add(XLinkNames::hrefAttr);
 }
-
-}