net: remove unused `var self = this` from old code

Removed an unused `var self = this` that is no longer required.

PR-URL: https://github.com/nodejs/node/pull/5224


Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>