[Microdata] HTMLPropertiesCollection code cleanup

[Microdata] HTMLPropertiesCollection code cleanup
https://bugs.webkit.org/show_bug.cgi?id=109721

Reviewed by Ryosuke Niwa.

Removed forward declaration of DOMStringList class.
Removed unused findRefElements() method declaration.
Also Removed unused parameter Element* from updatePropertyCache() method.

No new test since no change in behavior.

* html/HTMLPropertiesCollection.cpp:
(WebCore::HTMLPropertiesCollection::updateNameCache):
* html/HTMLPropertiesCollection.h:
(WebCore):
(HTMLPropertiesCollection):
(WebCore::HTMLPropertiesCollection::updatePropertyCache):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@142787 268f45cc-cd09-0410-ab3c-d52691b4dbfc