Remove "Primary Owner" from Owners

Summary:
Ref T8320. This field doesn't have any special effects and has no reason to exist. Primary owners are always already owners.

This makes one minor implicit change to Owners: it is now possible to create a package with no owners. That seems fine; it is reasonable to create a package purely as an organizational tool and use it in, e.g., Herald.

Test Plan: Created and edited packages.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T8320

Differential Revision: https://secure.phabricator.com/D13921