Remove some dead code from html/

Source/core/html/PluginDocument.cpp

 /*
  * Copyright (C) 2006, 2008 Apple 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:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 164 matching lines @@
     return m_pluginNode.get();
 }
 
 void PluginDocument::detach(const AttachContext& context)
 {
     // Release the plugin node so that we don't have a circular reference.
     m_pluginNode = nullptr;
     HTMLDocument::detach(context);
 }
 
-void PluginDocument::cancelManualPluginLoad()
-{
-    // PluginDocument::cancelManualPluginLoad should only be called once, but there are issues
-    // with how many times we call beforeload on object elements. <rdar://problem/8441094>.
-    if (!shouldLoadPluginManually())
-        return;
-
-    DocumentLoader* documentLoader = frame()->loader().documentLoader();
-    documentLoader->cancelMainResourceLoad(ResourceError::cancelledError(documentLoader->request().url()));
-    setShouldLoadPluginManually(false);
 }
-
-}