paymentrequest: don't show PaymentAck to user

mainly because the main "merchant" using bip70 is bitpay, and they
are failing all the PaymentAcks due to the tx is using RBF...
no need to confuse users.

follow-up 1686a97ece

electrum/gui/qt/main_window.py

@@ -1667,7 +1667,7 @@ class ElectrumWindow(QMainWindow, MessageBoxMixin, PrintError):
                 coro = pr.send_payment_and_receive_paymentack(str(tx), refund_address)
                 fut = asyncio.run_coroutine_threadsafe(coro, self.network.asyncio_loop)
                 ack_status, ack_msg = fut.result(timeout=20)
-                msg += f"\n\nPayment ACK: {ack_status}.\nAck message: {ack_msg}"
+                self.print_error(f"Payment ACK: {ack_status}. Ack message: {ack_msg}")
             return status, msg
 
         # Capture current TL window; override might be removed on return