Clean up various pieces of dead/obsolete Differential code master
Summary: Ref T2222. - Removes `DifferentialTasksAttacher`, which has had no callsites for a very long time. - Moves `differential.getrevisioncomments` off `DifferentialCommentQuery`. - Moves Releeph churn field off `DifferentialCommentQuery`. - Removes dead code in `DifferentialRevisionViewController`. - Removes `DifferentialException` (no references). - Removes `DifferentialRevision->loadComments()` (no callsites). - Removes `DifferentialRevision->loadReviewedBy()` (all callsites updated). - Removes `DifferentialCommentQuery` (all callsites updated). Test Plan: Mostly a lot of `grep`. Reviewers: btrahan Reviewed By: btrahan CC: aran Maniphest Tasks: T2222 Differential Revision: https://secure.phabricator.com/D8476