Remove ordered_objects from admin context.

Hi!

The true purpose of this pull-request is fix some django-1.6 incompatibility.

It removes get_ordered_objects call, because on django 1.6 this method does not exists and on django 1.5 it always return a empty list (https://github.com/django/django/blob/stable/1.5.x/django/db/models/options.py#L543)
Also, it seems not used by reversion but I'm not sure with this assertion!

With this change, django-reversion seems work properly with django-1.6beta1