[FIX] point_of_sale: prevent showing downpayment as discount on receipt

Current behavior:
When applying a discount and a downpayment on a product in the PoS
the discount line on the receipt would be the sum of the discount
and the downpayment. But the downpayment is not a discount.

Steps to reproduce:
- Install PoS
- Change discount policy to Show public price & discount to the
  customer, in the public pricelist
- Create a promotion program that applies 10% discount
- Use this promotion program in the PoS
- Create a quotation for any product (e.g with a product that cost
  1000$)
- In the PoS go in the quotation/order and apply a downpayment to
  this quotation.
- Pay the downpayment, and go in the quotation/order again. But this
  time settle the order.
- After the payment on the receipt you will see the discount line
  value is not correct. It's the sum of the discount + the downpayment

opw-2792417

closes odoo/odoo#89028

Signed-off-by: Masereel Pierre <pim@odoo.com>