buffer: remove deprecated buffer.get/.set methods

These have been deprecated since Apr 27, 2013, and the plan was to
remove them in "node v0.13".

buffer.get(index) is superseded by buffer[index].
buffer.set(index, value) is superseded by buffer[index] = value.

These have never been documented at any point in node's history.

PR-URL: https://github.com/nodejs/node/pull/4594
Fixes: https://github.com/nodejs/node/issues/4587


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>
Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Roman Reiss <me@silverwind.io>