pc-bios/s390-ccw/virtio-blkdev: Remove virtio_assume_scsi()

The virtio_assume_scsi() function is very questionable: First, it
is only called for virtio-blk, and not for virtio-scsi, so the naming
is already quite confusing. Second, it is called if we detected a
"invalid" IPL disk, trying to fix it by blindly setting a sector
size of 512. This of course won't work in most cases since disks
might have a different sector size for a reason.

Thus let's remove this strange function now. The calling code can
also be removed completely, since there is another spot in main.c
that does "IPL_assert(virtio_ipl_disk_is_valid(), ...)" to make
sure that we do not try to IPL from an invalid device.

Message-Id: <20220704111903.62400-6-thuth@redhat.com>
Reviewed-by: Eric Farman <farman@linux.ibm.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>