Clean up various pieces of dead/obsolete Differential code
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