build: remove mdb from io.js

After upgrading to a newer v8 mdb is pretty much broken - even if using
the latest updates from nodejs. If nodejs decides to update their v8 we
can give it another go, but for now it's better to remove it than have
it in our tree unsupported.

PR-URL: https://github.com/iojs/io.js/pull/1023


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Rod Vagg <rod@vagg.org>