test: remove AIX guard in fs-options-immutable

The fs watch test was not run on AIX till now because of the known
issue, https://github.com/nodejs/node/issues/5085. Now that it is
completed, this guard can be removed.

PR-URL: https://github.com/nodejs/node/pull/12687


Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Brian White <mscdex@mscdex.net>