R1:36343600c5c7

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…