s390x: remove s390-zipl.rom

s390x: remove s390-zipl.rom

This is an s390 boot rom which was used in s390-virtio machine.
but since commit 3538fb6f89dd9bb2e7e59de2bfad52a45321c744
"s390x: remove s390-virtio machine", this file isn't used.
The only place it is referenced in the code is an unused
define ZIPL_FILENAME.  There's also comment in hw/s390/ipl.c
which I'm modifying too, to refer to s390-ccw.img instead.

Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>
Acked-by: Cornelia Huck <cornelia.huck@de.ibm.com>