Update the `/agent/check/deregister/` API endpoint to return a 404
HTTP response code when an attempt is made to de-register a check ID
that does not exist on the agent.
This brings the behavior of /agent/check/deregister/ in line with the
behavior of /agent/service/deregister/ which was changed in #10632 to
similarly return a 404 when de-registering non-existent services.
Fixes#5821
