s390: remove WARN_ON for misc_deregister() failures

The previous change added WARN_ON() in misc_deregister().  So it is not
necessary to WARN_ON() misc_deregister() failure by callers.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>