http: remove duplicate export

5425e0dc switched to using
the module.exports pattern vs just exports, but left
a duplicate export around for OutgoingMessage.

PR-URL: https://github.com/nodejs/node/pull/17982


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Jon Moss <me@jonathanmoss.me>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>