Polish removal of conduit shield, including legacy stripping for phabricator on phabricator oauth scenarios

Summary: ...just in case that stuff happens in the "wild". also cleaned up the logic here since we no longer have the conduit conditionality.

Test Plan: made sure I didn't break JS on the site. reasoned about logic of my function and asking people PHP typing questions in job interviews.

Reviewers: epriestley, vrana

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T891

Differential Revision: https://secure.phabricator.com/D3269