src: remove experimental warning for inspect

* Removes "experimental" warning.

Fixes: https://github.com/nodejs/node/issues/11770
PR-URL: https://github.com/nodejs/node/pull/12352


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Alexey Orlenko <eaglexrlnk@gmail.com>
Reviewed-By: Eugene Ostroukhov <eostroukhov@google.com>
Reviewed-By: Ali Ijaz Sheikh <ofrobots@google.com>