istream/dechunk: eliminate "closing" check from DeferredEof()
istream/dechunk: eliminate "closing" check from DeferredEof()

The check is not needed when we make sure that defer_eof_event is not
scheduled.

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