scsi: qla4xxx: remvoe unnecessary condition check for mempool_destroy()

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

Signed-off-by: Chengguang Xu <cgxu519@gmx.com>
Acked-by: Manish Rangankar <Manish.Rangankar@cavium.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>