Remove unnecessary MutableStylePropertySet casts.
<http://webkit.org/b/115075>

Reviewed by Geoffrey Garen.

Get rid of some now-unneeded static_casts that were left from the StylePropertySet
refactoring last weekend.

* css/StylePropertySet.cpp:
(WebCore::MutableStylePropertySet::setProperty):
* editing/EditingStyle.cpp:
(WebCore::EditingStyle::triStateOfStyle):
(WebCore::EditingStyle::removeStyleFromRulesAndContext):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@149029 268f45cc-cd09-0410-ab3c-d52691b4dbfc