url: stop exporting originFor()

PR-URL: https://github.com/nodejs/node/pull/10955
Fixes: https://github.com/nodejs/node/issues/10800


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>