Removed django.db.models.options.Options.one_to_one_field.

The last use of it was removed in r9641 (it's internal API) and it's been
broken since r7477, as there's no longer a maximum of one OneToOneField per
model, so anything relying on it contained subtle bugs.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@9643 bcc190cf-cafb-0310-a4f2-bffc1f526a37