Remove all readers/writers for task "subpriority"
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.
Reviewed By: amckinley
Maniphest Tasks: T10333
Differential Revision: https://secure.phabricator.com/D20266