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 using it (if we ever do), we could figure out what an appropriate policy is in this case.

Test Plan: Browsed around Diffusion, grepped for affected "effective user" methods and found no callsites.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D20224