Remove all readers/writers for task "subpriority"

Summary:
Depends on D20265. Ref T10333. Now that neither task lists nor workboards use subpriority, we can remove all the readers and writers.

I'm not actually getting rid of the column data yet, but anticipate doing that in a future change.

Note that the subpriority algorithm (removed here) is possibly better than the "natural order" algorithm still in use. It's a bit more clever, and likely performs far fewer writes. I might make the "natural order" code use an algorithm more similar to the "subpriority" algorithm in the future.

Test Plan: Grepped for subpriority.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T10333

Differential Revision: https://secure.phabricator.com/D20266