buffer: cleanup CallbackInfo

Dynamic checks that CallbackInfo holds an ArrayBuffer handle can be
converted into compiler enforced checks. Removed unused code, and
other minor cleanup.

PR-URL: https://github.com/nodejs/node/pull/5204


Reviewed-By: bnoordhuis - Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: indutny - Fedor Indutny <fedor.indutny@gmail.com>