Remove obsolete "relationships" code from Differential

Summary:
Ref T10967. There have been two different ways to load reviewers for a while: needReviewerStatus() and needRelationships().

The needRelationships() stuff was a false start along time ago that didn't really go anywhere. I believe the idea was that we might want to load several different types of edges (subscribers, reviewers, etc) on lots of different types of objects. However, all that stuff pretty much ended up modularizing so that main Query classes did not need to know about it, so needRelationships() never got generalized or went anywhere.

A handful of things still use it, but get rid of them: they should either needReviewerStatus() to get reviewer info, or the ~3 callsites that care about subscribers can just load them directly.

Test Plan:

• Grepped for removed methods (needRelationships(), getReviewers(), getCCPHIDs(), etc).
• Browsed Diffusion, Differential.
• Called differential.query.

It's possible I missed some stuff, but it should mostly show up as super obvious fatals ("call needReviewerStatus() before getReviewerStatus()!").

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10967

Differential Revision: https://secure.phabricator.com/D17518