R1:592591e71538

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…