buffer: remove TODOs in `atob` / `btoa`

Refs: https://github.com/nodejs/node/pull/38433#issuecomment-828426932

PR-URL: https://github.com/nodejs/node/pull/38548


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Zijian Liu <lxxyxzj@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>