mempool_destroy has taken null pointer check into account. Remove the
redundant check.

Link: https://lore.kernel.org/r/20191026194712.GA22249@sauravSigned-off-by: NSaurav Girepunje <saurav.girepunje@gmail.com>
Reviewed-by: NJames Smart <james.smart@broadcom.com>
Signed-off-by: NMartin K. Petersen <martin.petersen@oracle.com>