Fixed #7517 -- Added code to remove Sites from the site cache when they are...

Fixed #7517 -- Added code to remove Sites from the site cache when they are deleted. Thanks to Marc Fargas for the patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@7724 bcc190cf-cafb-0310-a4f2-bffc1f526a37