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: jungejason, tuomaspelkonen, aran

Reviewed By: jungejason

CC: aran, jungejason

Differential Revision: 830