Do not copy supplier and customer moves of drop shipment

issue11890
review436131003
(grafted from a5d3a4d05b613d7c9ff497fc7475ccaa7117e43d)

--HG--
branch : 6.0

stock.py

@@ -385,6 +385,8 @@ class ShipmentDrop(Workflow, ModelSQL, ModelView):
         else:
             default = default.copy()
         default.setdefault('moves', None)
+        default.setdefault('supplier_moves', None)
+        default.setdefault('customer_moves', None)
         return super(ShipmentDrop, cls).copy(shipments, default=default)
 
     @classmethod