Remove old Countdown route

Summary: removes old phabricator.com/countdown/{id} route and code that uses that URL scheme

Test Plan: loaded phabricator.com/countdown, verified that generated links point to phabricator.com/CXXX

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T12524

Differential Revision: https://secure.phabricator.com/D17681