R1:67de714a2f85

Remove (most) support for 'differential.attach-task-class'

Summary:
After D814 and D829, you should be able to implement this logic in the
didWriteRevision() method of the field.

Note that the attacher is still referenced in
ConduitAPI_differential_updatetaskrevisionassoc_Method. This method should
probably be moved to facebook/ since it's pretty Facebook-specific.

No rush on any of this, it's not hurting anything.

Test Plan:
- Hit differential.getcommitmessage
- Ran 'arc diff'

Reviewers…