multi: use separate cltv expiry field from invoice

Now that the Invoice struct contains the decoded final cltv delta value,
the decoding of payment requests can be removed from the invoice
registry.

contractcourt/mock_registry_test.go

@@ -39,8 +39,8 @@ func (r *mockRegistry) NotifyExitHopHtlc(payHash lntypes.Hash,
 
 func (r *mockRegistry) HodlUnsubscribeAll(subscriber chan<- interface{}) {}
 
-func (r *mockRegistry) LookupInvoice(lntypes.Hash) (channeldb.Invoice, uint32,
+func (r *mockRegistry) LookupInvoice(lntypes.Hash) (channeldb.Invoice,
 	error) {
 
-	return channeldb.Invoice{}, 0, channeldb.ErrInvoiceNotFound
+	return channeldb.Invoice{}, channeldb.ErrInvoiceNotFound
 }