Remove caching of style sheets in IE6 (cache may be invalid when there are multiple modules)

Review at http://gwt-code-reviews.appspot.com/159804

Review by: jgw@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@7694 8db76d5a-ed1c-0410-87a9-c151d255dfc7