diag308 set has been available for many machine generations, and
alternative reipl code paths has not been exercised and seems to be
broken without noticing for a while now. So, cleaning up all obsolete
reipl methods except currently used ones, assuming that diag308 set
always works.

Also removing not longer needed reset callbacks.
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>