rhel-9: Drop system-id from the ISO definition

RHEL-9 seems to not be consistent when generating its ISOs, causing the system-id to be "LINUX" for the DVD ISOs, but to not be present in the "boot" ISOs.

Let's rely only in the Volume ID to identify RHEL-9 ISOs.

Signed-off-by: Fabiano Fidêncio fidencio@redhat.com