routing: report failures to mission control directly

As there is no more state kept in the payment session, failure reporting
can go straight to mission control.

routing/payment_lifecycle.go

@@ -343,7 +343,7 @@ func (p *paymentLifecycle) sendPaymentAttempt(firstHop lnwire.ShortChannelID,
 func (p *paymentLifecycle) handleSendError(sendErr error) error {
 
 	final, reason := p.router.processSendError(
-		p.paySession, &p.attempt.Route, sendErr,
+		&p.attempt.Route, sendErr,
 	)
 	if !final {
 		// Save the forwarding error so it can be returned if