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>

lib/internal/net.js

@@ -9,13 +9,6 @@ function isLegalPort(port) {
   return +port === (+port >>> 0) && port <= 0xFFFF;
 }
 
-
-function assertPort(port) {
-  if (typeof port !== 'undefined' && !isLegalPort(port))
-    throw new RangeError('"port" argument must be >= 0 and < 65536');
-}
-
 module.exports = {
-  isLegalPort,
-  assertPort
+  isLegalPort
 };