lib: remove the comment of base64 validation

Since there's a comment about the 'base64' validation needed, and this
will result in a breaking change, so this should be removed.

PR-URL: https://github.com/nodejs/node/pull/29201


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anto Aravinth <anto.aravinth.cse@gmail.com>