Fixed #15559 - distinct queries introduced by [15607] cause errors with some custom model fields

This patch just reverts [15607] until a more satisfying solution can be
found.

Refs #11707

git-svn-id: http://code.djangoproject.com/svn/django/trunk@15791 bcc190cf-cafb-0310-a4f2-bffc1f526a37