[FIX] point_of_sale: list price display after tax mapping

- In general settings:
    - Sales:
        - Activate discount
        - Activate Multiple Sale Prices with option computed from formula
    - Point of sale:
        - Activate Multiple Sale Prices with option computed from formula
    - Accounting:
        - Set show price tax-included
- Modify the Public Pricelist: Set to Global, price discount 50%
- Modify the sale tax to be included in price
- Create a 0% tax included in price
- Open fiscal position: Set auto-detect, replace sale 15% by 0%
- Open a product and set customer tax to sale 15%
- Open POS session configuration
    - Set fiscal position, tax included in price and public pricelist
- Add product "Pedal Bin"

You will see in strikethough text the original selling price with tax included,
while taxes should have been removed from the tax mapping.
Note that this is purely aesthetical, the total amount calculated is correct.
Fixing by calculating the price from the original selling price.

opw-2155779

closes odoo/odoo#43259

Signed-off-by: Nicolas Lempereur (nle) <nle@odoo.com>