Drop differential_relationship table

Summary: Ref T2222. I'll hold this, but there are no more reads or writes from/to this table in the application.

Test Plan: Grepped for usage, ran migration, browsed around.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley, aran

Maniphest Tasks: T2222

Differential Revision: https://secure.phabricator.com/D8203