Remove DifferentialCommentEditor and DifferentialCommentMail

Summary: Ref T2222. These no longer have any callsites. Also got rid of a little bit of other code which also no longer has callsites.

Test Plan: `grep`

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2222

Differential Revision: https://secure.phabricator.com/D8443