Countdown has a Delete Countdown action, which completely destroys countdowns. This was written circa 400 BC.

In almost all cases, we've moved away from letting users permanently destroy data from the web UI: if a user makes a mistake, or an attacker compromises an account, it should generally be possible to undo and review any changes. Destroying objects also breaks integrations, like embedding countdowns in remarkup with {C123}.

Other applications support a "Disable" action, but I'm not sure there's actually any value in disabling countdowns, so let's just omit this until a use case arises.

• Remove the "Delete Countdown" action from the UI.
• Remove the "CountdownDeleteController".
• Remove the route in "CountdownApplication".
• Implement PhabricatorDestructibleInterface on PhabricatorCountdown.
• Verify that countdowns can be destroyed with ./bin/remove destroy <phid>.