net: remove an unused internal module `assertPort`

Since a module assertPort in lib/internal/net.js is not used anymore, we can remove it. The coverage of lib/internal/net.js will be 100% with this.

• https://coverage.nodejs.org/coverage-055482c21e2df944/root/internal/net.js.html

See also: https://github.com/nodejs/node/pull/11667

Checklist

• make -j4 test
• commit message follows commit guidelines

Affected core subsystem(s)

net