doc: update labels info in onboarding-extras.md

Remove information duplicated in onboarding.md. Organize label list for
clarity. Remove `deps` label which we don't actually have.

PR-URL: https://github.com/nodejs/node/pull/19160


Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>