s390/kexec: move machine_crash_shutdown() to machine_kexec.c

machine_crash_shutdown() was the only function in crash.c.
So move the function and delete one file.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>