scsi: libfc: 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>
Acked-by: Johannes Thumshirn <jth@kernel.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>