Discards the jar file name in the resource location.  It isn't necessary, and
will cause detritus to build up in the cache if a resource changes from being
in one jar to another or moves in/out of a .jar file.

Review at http://gwt-code-reviews.appspot.com/1428805


git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@10130 8db76d5a-ed1c-0410-87a9-c151d255dfc7