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