Remove obsolete "PhabricatorAuthLoginHandler"

Summary: Depends on D20096. Reverts D14057. This was added for Phacility use cases in D14057 but never used. It is obsoleted by Auth → Customize Messages for non-Phacility use cases.

Test Plan: Grepped for removed symbol.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D20099