s390/dasd: remove unnecessary condition check

kmem_cache_destroy() can handle NULL pointer correctly, so there is
no need to check NULL pointer before calling kmem_cache_destroy().

Signed-off-by: Chengguang Xu <cgxu519@gmx.com>
Signed-off-by: Jan Höppner <hoeppner@linux.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>