Removes utxo field from non-receiver inputs

Prior to this commit, the payjoin receiver code
was signing a PSBT containing the utxo field
for every input, including the ones it did not
own, and transferring this to the sender.
However BIP78 specifies that, for inputs belonging
to the sender, no utxo field should be included.
This is corrected in this commit.

jmclient/jmclient/payjoin.py

@@ -953,6 +953,8 @@ class PayjoinServer(Resource):
         # intended (note the outputs will be shuffled next!):
         outs[1]["value"] -= our_fee_bump
 
+        # TODO this only works for 2 input transactions, otherwise
+        # pure-shuffle will not be valid as per BIP78 ordering requirement.
         unsigned_payjoin_tx = btc.make_shuffled_tx(payjoin_tx_inputs, outs,
                                     version=payment_psbt.unsigned_tx.nVersion,
                                     locktime=payment_psbt.unsigned_tx.nLockTime)
@@ -970,6 +972,7 @@ class PayjoinServer(Resource):
                     inp.nSequence = inp2.nSequence
                     spent_outs.append(payment_psbt.inputs[j].utxo)
                     input_found = True
+                    sender_index = i
                     break
             if input_found:
                 continue
@@ -1007,6 +1010,11 @@ class PayjoinServer(Resource):
         assert signresult.num_inputs_final == len(receiver_utxos)
         assert not signresult.is_final
 
+        # with signing succcessful, remove the utxo field from the
+        # counterparty's input (this is required by BIP78). Note we don't
+        # do this on PSBT creation as the psbt signing code throws ValueError
+        # unless utxos are present.
+        receiver_signed_psbt.inputs[sender_index] = btc.PSBT_Input(index=sender_index)
         log.debug("Receiver signing successful. Payjoin PSBT is now:\n{}".format(
             self.wallet_service.human_readable_psbt(receiver_signed_psbt)))
         # construct txoutset for the wallet service callback; we cannot use