buffer: remove raw & raws encoding

As `raw` and `raws` encodings are deprecated for such a long time, and
they both are undocumented, this patch removes the support for those
encodings completely.

Previous discussion: https://github.com/nodejs/node/pull/2829

PR-URL: https://github.com/nodejs/node/pull/2859


Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>