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/11859
Refs: c2a302c5 "src: do not ignore IDNA conversion error"
Refs: https://url.spec.whatwg.org/#concept-host-parser


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Daijiro Wachi <daijiro.wachi@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>