Remove "Destroy" action for Countdown objects

Summary: fixes T12523

Test Plan:

• view Countdown edit screen, Destroy action missing
• checked that ./bin/remove destroy <some-countdown-phid> removes the DB rows as expected

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T12523

Differential Revision: https://secure.phabricator.com/D17659