R1:99e5ef84fc25

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 {nav 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