fs: use errno constant with ERR_FS_EISDIR

This commit updates rm() to use the EISDIR constant with
ERR_FS_EISDIR instead of hard coding -21.

PR-URL: https://github.com/nodejs/node/pull/35563


Reviewed-By: Ben Coe <bencoe@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Richard Lau <rlau@redhat.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>