istream/delayed: remove redundant DeferEvent::Cancel() call from destructor
istream/delayed: remove redundant DeferEvent::Cancel() call from destructor

DeferEvent's hook is `auto_unlink`.

CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy