s390: remove dead code

Remove some dead code from head64.S, which was left over since commit
da292bbe

 ("[S390] eliminate ipl_device from lowcore") removed
ipl_device from lowcore.

Reviewed-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Gerald Schaefer <gerald.schaefer@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>