Remove old reviewer double writes to legacy edge table in Differential

Summary:
Ref T2543. Ref T10967. This isn't precisely related to "draft" status, but while I'm churning this stuff anyway, get rid of the old double writes to clean the code up a bit.

These were added in T10967 to make sure the migration was reversible/recoverable, but we haven't seen any issues with it in several months so I believe they can now be removed safely. Nothing has read this table since ~April.

Test Plan: Took various review actions on revisions (accept, reject, resign, comment, etc). If this change is correct, there should be no visible effect.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10967, T2543

Differential Revision: https://secure.phabricator.com/D18398