R1:74a0caf9ce16

Remove "Author" CustomField in Differential

Summary: Ref T11114. This hasn't done anything since we moved author information to the subheader.

Test Plan: Browsed Differential, still saw author information.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11114

Differential Revision: https://secure.phabricator.com/D17071