Remove almost all old Differential field code

Summary: Ref T2222. The unit and lint fields still have one piece of functionality that I need to port, but everythign else is obsolete.

Test Plan: Lots of grep.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2222

Differential Revision: https://secure.phabricator.com/D8474