Fix failure to abandon async payments on invalid static invoice

Prior to this fix, we would attempt to mark outbound async payments as
abandoned but silently fail because they were in state AwaitingInvoice, which
the mark_abandoned utility doesn't currently work for. These payments would
eventually be removed by the remove_stale_payments method, but there would be a
delay in generating the PaymentFailed event.

Move to manually removing the outbound payment entry.

Author: valentinewallace

lightning/src/ln/outbound_payment.rs

@@ -1028,17 +1028,13 @@ impl OutboundPayments {
 	) -> Result<(), Bolt12PaymentError> where ES::Target: EntropySource {
 		macro_rules! abandon_with_entry {
 			($payment: expr, $reason: expr) => {
-				$payment.get_mut().mark_abandoned($reason);
-				if let PendingOutboundPayment::Abandoned { reason, .. } = $payment.get() {
-					if $payment.get().remaining_parts() == 0 {
-						pending_events.lock().unwrap().push_back((events::Event::PaymentFailed {
-							payment_id,
-							payment_hash: None,
-							reason: *reason,
-						}, None));
-						$payment.remove();
-					}
-				}
+				debug_assert!(matches!($payment.get(), PendingOutboundPayment::AwaitingInvoice { .. }));
+				pending_events.lock().unwrap().push_back((events::Event::PaymentFailed {
+					payment_id,
+					payment_hash: None,
+					reason: Some($reason),
+				}, None));
+				$payment.remove();
 			}
 		}