s390/ipl: remove non-existing functions declaration

do_reipl, do_halt and do_poff are not defined anywhere. Cleaning up
functions declaration.

Reviewed-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>