Simplify `Transfer::transfer` return type

We never use the fee returned from this function, remove it.

swap/src/monero.rs

@@ -190,7 +190,7 @@ pub trait Transfer {
         public_spend_key: PublicKey,
         public_view_key: PublicViewKey,
         amount: Amount,
-    ) -> Result<(TransferProof, Amount)>;
+    ) -> Result<TransferProof>;
 }
 
 #[async_trait]

swap/src/monero/wallet.rs

@@ -82,7 +82,7 @@ impl Transfer for Wallet {
         public_spend_key: PublicKey,
         public_view_key: PublicViewKey,
         amount: Amount,
-    ) -> Result<(TransferProof, Amount)> {
+    ) -> Result<TransferProof> {
         let destination_address =
             Address::standard(self.network, public_spend_key, public_view_key.into());
 
@@ -97,12 +97,10 @@ impl Transfer for Wallet {
         tracing::info!("Monero tx broadcasted!, tx hash: {:?}", tx_hash);
         let tx_key = PrivateKey::from_str(&res.tx_key)?;
 
-        let fee = Amount::from_piconero(res.fee);
-
         let transfer_proof = TransferProof::new(tx_hash, tx_key);
         tracing::debug!("  Transfer proof: {:?}", transfer_proof);
 
-        Ok((transfer_proof, fee))
+        Ok(transfer_proof)
     }
 }
 

swap/src/protocol/alice/steps.rs

@@ -69,7 +69,7 @@ where
     let public_spend_key = S_a + state3.S_b_monero;
     let public_view_key = state3.v.public();
 
-    let (transfer_proof, _) = monero_wallet
+    let transfer_proof = monero_wallet
         .transfer(public_spend_key, public_view_key, state3.xmr)
         .await?;