Remove `buffer.get` and `buffer.set`

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

buffer.get(index) is superseded by buffer[index].

buffer.set(index, value) is superseded by buffer[index] = value.

I haven't seen these used in real code, ever. They also have never been documented at any point in node's history, according to the git commit history for doc/api/buffer.markdown.

I think it is safe to finally remove these.

Thoughts?