doc: remove Caine section from contributing guide

Caine bot is not active on the repository anymore.
This section is just confusing for new contributors.

Fixes: https://github.com/iojs/io.js/issues/802
PR-URL: https://github.com/iojs/io.js/pull/804


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>