R1:1d90577d8c1c

Drop old Differential custom field storage

Summary: Ref T2222. See D8355. I'll hold this for a while.

Test Plan: Ran migration.

Reviewers: chad, btrahan

Reviewed By: chad

Subscribers: epriestley, aran

Maniphest Tasks: T2222

Differential Revision: https://secure.phabricator.com/D8356