mixpool: Remove ban for invalid UTXO check

None of the existing UTXO checks result in an automatic ban, so avoid this
behavior for an invalid signature proof as well.

mixing/mixpool/errors.go

@@ -76,10 +76,6 @@ var (
 	// that exceeds the total input value.
 	ErrInvalidTotalMixAmount = newBannableError("invalid total mix amount")
 
-	// ErrInvalidUTXOProof is returned by AcceptMessage if a pair request
-	// fails to prove ownership of each utxo.
-	ErrInvalidUTXOProof = newBannableError("invalid UTXO ownership proof")
-
 	// ErrMissingUTXOs is returned by AcceptMessage if a pair request
 	// message does not reference any previous UTXOs.
 	ErrMissingUTXOs = newBannableError("pair request contains no UTXOs")

mixing/mixpool/mixpool.go

@@ -1313,7 +1313,7 @@ func (p *Pool) checkUTXOs(pr *wire.MsgMixPairReq, curHeight int64) error {
 		valid := validateOwnerProofP2PKHv0(extractPubKeyHash160,
 			entry.PkScript(), utxo.PubKey, utxo.Signature, pr.Expires())
 		if !valid {
-			return ruleError(ErrInvalidUTXOProof)
+			return ruleError(fmt.Errorf("invalid UTXO ownership proof"))
 		}
 
 		totalValue += entry.Amount()