R1:4df072cca688

Remove "DifferentialEditPolicyField" custom field

Summary: Ref T11114. This is now entirely handled by EditEngine and standard policy code.

Test Plan: Edited the edit policy of a revision.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11114

Differential Revision: https://secure.phabricator.com/D17074