Remove Certificate page

Summary: Closes T9703. This page has become redundant 10 months ago, at D10988.

Test Plan: Look at /settings page, don't see word "Certificate".

Reviewers: epriestley, #blessed_reviewers, chad

Reviewed By: #blessed_reviewers, chad

Subscribers: Korvin

Maniphest Tasks: T9703

Differential Revision: https://secure.phabricator.com/D14400