s390x: remove s390-zipl.rom

This is an s390 boot rom which was used in s390-virtio machine.
but since commit 3538fb6f89
"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>

pc-bios/README

@@ -35,10 +35,6 @@
 	10ec:8139 -> pxe-rtl8139.rom
 	1af4:1000 -> pxe-virtio.rom
 
-- The S390 zipl loader is an addition to the official IBM s390-tools
-  package. That fork is maintained in its own git repository at:
-  git://repo.or.cz/s390-tools.git
-
 - The sources for the Alpha palcode image is available from:
   git://github.com/rth7680/qemu-palcode.git