[1.0.X] Fixed #2698 -- Fixed deleting in the presence of custom managers.

A custom manager on a related object that filtered away objects would prevent
those objects being deleted via the relation. This is now fixed.

Backport of r10104 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.0.X@10106 bcc190cf-cafb-0310-a4f2-bffc1f526a37