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 The unit and lint fields still have one piece of functionality that I need to port, but everythign else is obsolete.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