Fixed #10589 -- Removed duplicated model definition in null_fk_ordering...

Fixed #10589 -- Removed duplicated model definition in null_fk_ordering regression test. Thanks to Julien Phalip for the report.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@10143 bcc190cf-cafb-0310-a4f2-bffc1f526a37