Remove isPluginElement hack in Document::setFocusedNode()
<http://webkit.org/b/115171>

From Blink r148800 by <tkent@chromium.org>:

This hack is no longer necessary because r147591 made isFocusable for plugin
elements work correctly.

* dom/Document.cpp:
(WebCore::Document::setFocusedNode):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@149101 268f45cc-cd09-0410-ab3c-d52691b4dbfc