Remove `differential.markcommitted`

Summary: Ref T2222. Primary goal is to remove this callsite for `DifferentialCommentEditor`, but rather than updating it I'm just nuking this method since it's been deprecated for more than a year (more than two years?)

Test Plan: Reloaded Conduit method list.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2222

Differential Revision: https://secure.phabricator.com/D8424