Fixed #17918 - Handle proxy models correctly when sorting deletions for...

Fixed #17918 - Handle proxy models correctly when sorting deletions for databases without deferred constraints. Thanks Nate Bragg for the report.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@17756 bcc190cf-cafb-0310-a4f2-bffc1f526a37