[FIX] point_of_sale: avoid adding the same tax twice

Activate a Fiscal position with a defined tax mapping(i.e. 15%). Add to a
product the same tax. Set the tax to be included in price.
In POS Settings add the default fiscal position as the one from before.
Now open POS and add the product.

The total price calculated will be wrong: the tax is added twice, one
from the product line and one from the localization.

Mimic the behavior as in
https://github.com/odoo/odoo/blob/saas-12.3/addons/account/models/account.py#L1512



closes odoo/odoo#42180

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