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