Fixed #8999 -- Removed useless code in handling of `exclude` option in ModelAdmin and InlineModelAdmin custom form(set) hooks. Thanks goes to seanl for the report, patch and bkonkle and Julien for further work on final patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@16422 bcc190cf-cafb-0310-a4f2-bffc1f526a37