net: refactor onSlaveClose in Server.close

Refactors onSlaveClose in Server.close to be an arrow function,
removes need for `self = this` and moves it down to make code
more readable.

PR-URL: https://github.com/nodejs/node/pull/12334


Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>