Removed all parent_*_obj_get functions.

DETAILS:
Removed on the grounds that the Tastypie authorization change has
rendered it obsolete.

If the nested object list needed to do some extra authorization stuff,
it should do so within their own Authorization object by checking the
parent_resource and parent_object in bundle.request.kwargs. (which I may
not have a proper implementation yet, will think it through first --
after all it's still a development WIP until unit tests are done :)).