Remove obsolete code from ManiphestTaskQuery

Summary:
Ref T603. Cleans up some obsolete stuff here:

  - We no longer ever query by min/max priority (instead, `withPriorities(...)`).
  - A parent class provides limit/offset.
  - Result count is no longer reliable with policies. We could do "about X tasks" or something, but just drop it for now. There's only one remaining callsite anyway.

Test Plan:
  - `grep`
  - Viewed task list.
  - Viewed a project page.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T603

Differential Revision: https://secure.phabricator.com/D7121