Remove newly-output PersistentUnitCache file if nothing was written to it.

In the case where you re-run a compile/startup without changing any code, we produce an empty cache file each time.  This change removes the file on exit if we never wrote anything to it.

http://gwt-code-reviews.appspot.com/1396804/

Review by: zundel@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@9921 8db76d5a-ed1c-0410-87a9-c151d255dfc7