cpu: Remove parameter of list_cpus()

The "optarg" parameter is completely unused, so let's drop it.

Message-Id: <20230419124831.678079-1-thuth@redhat.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>