R1:d179d0150c4c

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…