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