Force Differential draft uniqueness

Summary: Ref T1191. A couple of installs have hit issues with this table, so clean it up before adjustment adds a unique key to it.

Test Plan: Dropped key, added duplicate rows, ran patch, got cleanup, ran adjust to get the key back.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T1191

Differential Revision: https://secure.phabricator.com/D10799