R1:84572a3b9351

Remove Differential subscribers field

Summary: Ref T11114. This is obsoleted by `DifferentialSubscribersCommitMessageField` and EditEngine.

Test Plan: Edited a revision's subscribers.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11114

Differential Revision: https://secure.phabricator.com/D17081