[#326] ir/settlement: Move the calculation of the minimum audit fee

Move control of the minimum cost of payment for the audit from the
implementation of the Exchanger to the place where the amount is calculated.

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

pkg/innerring/audit.go

@@ -175,13 +175,8 @@ func (a auditSettlementDeps) Transfer(sender, recipient *owner.ID, amount *big.I
 		return
 	}
 
-	amount64 := amount.Int64()
-	if amount64 == 0 {
-		amount64 = 1
-	}
-
 	if err := a.balanceClient.TransferX(balanceClient.TransferPrm{
-		Amount:  amount64,
+		Amount:  amount.Int64(),
 		From:    sender,
 		To:      recipient,
 		Details: transferAuditDetails,