Remove the Persona login method

Summary:
Persona is going to be decommed November 30th, 2016.
It is highly unlikely that anyone is currently using persona as a real
login method at this point.

Test Plan: tried locally to add auth adapter.

Reviewers: chad, Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D16371