net: remove an unused internal module `assertPort`

A module `assertPort` in `lib/internal/net.js` is not used anymore.

Refs: https://github.com/nodejs/node/pull/11667
PR-URL: https://github.com/nodejs/node/pull/11812


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>