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

src/applications/maniphest/query/ManiphestTaskSearchEngine.php

@@ -236,8 +236,6 @@ final class ManiphestTaskSearchEngine
     $group = idx($this->getGroupValues(), $group);
     if ($group) {
       $query->setGroupBy($group);
-    } else {
-      $query->setGroupBy(head($this->getGroupValues()));
     }
 
     if ($map['ids']) {