Remove setOwners() from PhabricatorProjectQuery

Summary: I'm deprecating the concept of "owners" (which currently has no meaning in the actual application) in favor of policy-based controls. Remove the ability to query by it.

Test Plan: Grepped for setOwners(), no relevant hits. Browsed the project listing.

Reviewers: vrana, btrahan

Reviewed By: vrana

CC: aran

Maniphest Tasks: T603

Differential Revision: https://secure.phabricator.com/D3178