[FIX] point_of_sale: stop showing changes of fiscal position as discount

Current behavior:
When a fiscal position was applied in the PoS, it was considered as a
discount and was shown in the receipt and on the product screen.

Steps to reproduce:
- Create a product with a price of 10€ with 15% tax included in the
  price
- Create a fiscal position that map the 15% tax to 0% tax
- Open a PoS session
- Add the product to the order
- Apply the fiscal position
- The price of the product is now 8.70€ but the order show it as a
  discount of 1.30€ (e.g 10€ -> 8.70€).

opw-3154499

closes odoo/odoo#116053

X-original-commit: bbccf240f0dae32694ca2ff69fc180ab12bd932b
Signed-off-by: Trinh Jacky (trj) <trj@odoo.com>
Signed-off-by: Engels Robin (roen) <roen@odoo.com>