doc: remove repeated info onboarding.md

COLLABORATOR_GUIDE.md and onboarding.md cover some of the same
information. The aim of this commit is to remove duplicated
information.

PR-URL: https://github.com/nodejs/node/pull/9635


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>