R1:d6e47eef19f4

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…