Don't set default unit price on stock move

issue8119
review255251002

tests/scenario_carrier_weight.rst

@@ -131,8 +131,7 @@ Receive a single product line::
     >>> move.to_location = shipment.warehouse.input_location
     >>> move.product = product
     >>> move.quantity = 4
-    >>> move.unit_price
-    Decimal('8')
+    >>> move.unit_price = Decimal('8')
     >>> shipment.carrier = carrier
     >>> shipment.cost
     Decimal('25')