fs: remove experimental from promises.rmdir recursive

This was missed in commit 35b17d9.

Refs: #34278
Refs: #35171

Signed-off-by: Anders Kaseorg <andersk@mit.edu>

PR-URL: #36131
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Ben Coe <bencoe@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

Author: andersk

doc/api/fs.md

@@ -5614,8 +5614,6 @@ changes:
                   now supported.
 -->
 
-> Stability: 1 - Recursive removal is experimental.
-
 * `path` {string|Buffer|URL}
 * `options` {Object}
   * `maxRetries` {integer} If an `EBUSY`, `EMFILE`, `ENFILE`, `ENOTEMPTY`, or