[S390] Remove unneeded spinlock initialization.

Remove the now unneeded s390_idle.lock spinlock initialization after
Josef Sipek did it the right way in arch/s390/kernel/process.c.

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