doc: Remove 'dnt_helper.js'

This file is totally useless, because it's an inner-used helper that is
for publishment of doc files.

PR-URL: https://github.com/nodejs/node/pull/22595


Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>