http: remove duplicate export

5425e0d switched to using
the module.exports pattern vs just exports, but left
a duplicate export around for OutgoingMessage.

PR-URL: nodejs#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>

lib/_http_outgoing.js

@@ -179,9 +179,6 @@ OutgoingMessage.prototype._renderHeaders = function _renderHeaders() {
 };
 
 
-exports.OutgoingMessage = OutgoingMessage;
-
-
 OutgoingMessage.prototype.setTimeout = function setTimeout(msecs, callback) {
 
   if (callback) {