R1:c116deef6398

Remove "Effective User" attachment from Repository Identities

Summary:
See <https://discourse.phabricator-community.org/t/phabricatorrepositoryidentity-attacheffectiveuser-must-be-an-instance-of-phabricatoruser-null-given/1820/3>.

It's possible for an Idenitity to be bound to user X, then for that user to be deleted, e.g. with `bin/remove destroy X`. In this case, we'll fail to load/attach the user to the identity.

Currently, we don't actually use this anywhere, so just stop loading it. Once we start…