remove phanthom tables

Addresses the issue that https://www.pivotaltracker.com/story/show/83759162 was attempting to prevent.

Upon further investigation, we've determine that it was not a faulty migration that was causing "ghost" tables from returning. The problem was `author_groups` and `message_participants` table was still on staging despite the fact that `db:migrate:status` indicates the the migration deleting those tables have been executed.

This is a stop gap to prevent this error from propagating to other developer machines.