Remove references to nonexistent PhabricatorOAuthProviderPhabricator

Summary:
This gets added in D1595 (which hasn't landed yet), but was referred to in
D1632 (already committed). This unbreaks master for me.

Test Plan: I no longer get an error trying to load
PhabricatorOAuthProviderPhabricator

Reviewers: btrahan, epriestley

Reviewed By: epriestley

CC: jungejason, aran, epriestley

Differential Revision: https://secure.phabricator.com/D1633