n-api: remove experimental gate from status codes

Re: https://github.com/nodejs/node/pull/21226#discussion_r200534431
PR-URL: https://github.com/nodejs/node/pull/21680


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gus Caplan <me@gus.host>