[FIX] point_of_sale,pos_{sale,gift_card}: product taken in total discount

When showing the discount the to the public, the down payment and gift card products were taken into account in the discount computation shown in the receipt.

This also fix a mistake made in fc546e04 where it was implemented in the wrong place and returning a wrong value in a reduce callback.

closes odoo/odoo#89603

X-original-commit: 597ea7aa3f28ad82e602e1ba5679bb49d2973b78
Signed-off-by: Masereel Pierre <pim@odoo.com>
Signed-off-by: Trinh Jacky (trj) <trj@odoo.com>