Unrequire filling Owners in Owners tool

Summary:
Owners field is filled by Primary Owner which is required.
So that it is not neccessary to require filling Owners explicitly.

Test Plan: Don't fill Owners and successfully save the form before this change.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran

Differential Revision: https://secure.phabricator.com/D2201