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: #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>

lib/internal/validators.js

@@ -151,8 +151,6 @@ function validateEncoding(data, encoding) {
     throw new ERR_INVALID_ARG_VALUE('encoding', encoding,
                                     `is invalid for data of length ${length}`);
   }
-
-  // TODO(bnoordhuis) Add BASE64 check?
 }
 
 module.exports = {