Simplify Auditors custom field in Differential

Summary: Ref T11114. This field just stores the value of "Auditors" so you can trigger auditors explicitly later on if you want.

Test Plan: Created and edited revisions with "Auditors".

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11114

Differential Revision: https://secure.phabricator.com/D17070