Remove old reviewer double writes to legacy edge table in Differential
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.
Reviewed By: chad
Differential Revision: https://secure.phabricator.com/D18398