Stop writes to the old Relationship table

Summary: Ref T1279. The new stuff seems stable, so stop writes to the old tables.

Test Plan:
  - Added and removed reviewers.
  - Grepped for `::RELATIONSHIP_TABLE` to verify we really have no more reads.
  - Grepped for `::RELATION_REVIEWER`.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1279

Differential Revision: https://secure.phabricator.com/D7360