Remove shipment cost method if there is no carrier for web sale

Since 52a777d4e4d0 the shipment cost method is no more removed by on_change's.

Closes #12546

modules/web_shop_vue_storefront/sale.py

@@ -168,6 +168,8 @@ class SaleShipmentCost(metaclass=PoolMeta):
                 carrier = None
             # Use id to get proper context
             self.carrier = carrier.id if carrier else None
+            if not self.carrier:
+                self.shipment_cost_method = None
 
 
 class Line(metaclass=PoolMeta):