s390/ipl: remove reipl_method and dump_method

reipl_method and dump_method have been used in addition to reipl_type
and dump_type, because a single reipl_type could be achieved with
multiple reipl_method (same for dump_type/method). After dropping
non-diag308_set based reipl methods, there is a single method per
reipl_type/dump_type and reipl_method and dump_method could be simply
removed.

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>