[1.2.X] Fixed #14707 -- Relax the protections on aggregate naming collisions...

[1.2.X] Fixed #14707 -- Relax the protections on aggregate naming collisions when a ValuesQuerySet removes the colliding name. Thanks to Andy McKay for the report.

Backport of r15223 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@15224 bcc190cf-cafb-0310-a4f2-bffc1f526a37