Drop old expiry_time_from_unix_epoch helper in expiry time lookup

Since there's a much simpler way to go about it with
`Bolt11Invoice::expires_at`.

Author: TheBlueMatt

lightning-invoice/src/payment.rs

@@ -61,10 +61,6 @@ pub fn payment_parameters_from_invoice(invoice: &Bolt11Invoice)
 	}
 }
 
-fn expiry_time_from_unix_epoch(invoice: &Bolt11Invoice) -> Duration {
-	invoice.signed_invoice.raw_invoice.data.timestamp.0 + invoice.expiry_time()
-}
-
 fn params_from_invoice(invoice: &Bolt11Invoice, amount_msat: u64)
 -> (PaymentHash, RecipientOnionFields, RouteParameters) {
 	let payment_hash = PaymentHash((*invoice.payment_hash()).into_inner());
@@ -76,8 +72,10 @@ fn params_from_invoice(invoice: &Bolt11Invoice, amount_msat: u64)
 			invoice.recover_payee_pub_key(),
 			invoice.min_final_cltv_expiry_delta() as u32
 		)
-		.with_expiry_time(expiry_time_from_unix_epoch(invoice).as_secs())
 		.with_route_hints(invoice.route_hints()).unwrap();
+	if let Some(expiry) = invoice.expires_at() {
+		payment_params = payment_params.with_expiry_time(expiry);
+	}
 	if let Some(features) = invoice.features() {
 		payment_params = payment_params.with_bolt11_features(features.clone()).unwrap();
 	}