doc: remove Locked from stability index

The stability index 3 (Locked) is unused and is being eliminated. Remove
it from the documentation about the stability index.

Remove mention of the Locked from CONTRIBUTING.md. The remaining text
about the stability index is slight and not seemingly valuable. Removing
it too.

PR-URL: https://github.com/nodejs/node/pull/11661
Ref: https://github.com/nodejs/node/issues/11200


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>