Fixed #12953 -- Ensure that deletion cascades through generic relations. Also...

Fixed #12953 -- Ensure that deletion cascades through generic relations. Also cleans up the special-casing of generic relations in the deleted object discovery process. Thanks to carljm for the report and patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@12790 bcc190cf-cafb-0310-a4f2-bffc1f526a37