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.
I think there is stronger backwards-compatibility rationale for keeping the `.related` attribute than for keeping `RelatedObject` itself. I see (and write) a lot of code that accesses various properties of relation descriptors for introspection, but I haven't seen so much code importing and using `RelatedObject` directly. That said, a GitHub code search shows that there is quite a bit of such code out there: https://github.com/search?l=python&p=2&q=RelatedObject&type=Code&utf8=%E2%9C%93

I do feel like since we have a clear policy on what code our backwards-compatibility policy applies to, and clearly none of this code falls under it, we shouldn't go very far out of our way to preserve backwards-compat.