Don't set a default "group by priority" in the task search engine

Summary:
See PHI42. Currently, maniphest.search incorrectly applies this default (group by priority) to all queries via Conduit.

The correct behavior is to apply no grouping constraint.

I think this is also a reasonable general behavior, and the current code seems to date from D6960 in 2013 and didn't seem particularly carefully considered.

This is a minor compatibility break -- saved queries which are more than 4 years old might change their group behavior. I'll note this in the change logs but expect essentially no one to be affected.

Test Plan: Ran a maniphest.search Conduit call and observed the underlying query. Before this change, it executed ORDER BY priority, id. After this change, it correctly executed ORDER BY id only.

Reviewers: chad

Reviewed By: chad

Differential Revision: https://secure.phabricator.com/D18459