R1:9c423647d121

Drop DifferentialComment and DifferentialInlineComment

Summary: Ref T2222. No remaining readers or writers. Holding for a while.

Test Plan: `grep`

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley, aran

Maniphest Tasks: T2222

Differential Revision: https://secure.phabricator.com/D8242