Fixed #21414 -- Removed usage of RelatedObject and deprecated it.

The django.db.models.related module is now deprecated. The functionality
was moved to ForeignObjectRel which is accessible from field.rel.
RelatedObject was accessible from field.related.
@akaariai - do you think we need to keep the code in `RelatedObject` -- no tests fail if I make it a stub class like this:
```
class RelatedObject(six.with_metaclass(InstanceCheckMeta)):
    pass
```
(I am not sure what the backwards compatibility rationale for keeping it is.) Seems like we should have some tests for it if we do need to keep it.