[1.2.X] Fixed #13730 -- Removed the hard-coding of the requirement that...

[1.2.X] Fixed #13730 -- Removed the hard-coding of the requirement that ForeignKeys have an index. Thanks to Suor for the report.

Backport of r13452 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@13454 bcc190cf-cafb-0310-a4f2-bffc1f526a37