[FIX] point_of_sale: recompute reward after pricelist change

Current behavior:
In the PoS if you apply a coupon on an order that contains 2 products
with different taxes, it will create 2 discount lines on the order.
If you change the pricelist, the discount lines won't have the correct
values.

Steps to reproduce:
- Create 2 different products with different taxes.
- Create a pricelist with a discount on the 2 products.
- Create a coupon that apply a 100% discount on the order.
- Start a PoS session
- Add the 2 products to the order and apply the coupon.
- The order total is now 0€.
- Change the pricelist to the one with the discount.
- The order total is different than 0€.

opw-3049098

closes odoo/odoo#106067

Signed-off-by: Trinh Jacky (trj) <trj@odoo.com>