src: remove explicit UTF-8 validity check in url

This step was never part of the URL Standard's host parser algorithm,
and is rendered unnecessary after IDNA errors are no longer ignored.

PR-URL: https://github.com/nodejs/node/pull/12507
Refs: c2a302c5

 "src: do not ignore IDNA conversion error"
Reviewed-By: James M Snell <jasnell@gmail.com>