[FIX] point_of_sale: allow negative prices again
[FIX] point_of_sale: allow negative prices again

5afb5ecd04cf49be6bedb655b187b19a65e1e02d rewrote the tax computation
in both account and point_of_sale to fix complex tax computations but
it no longer allowed negative prices in the point of sale.

opw-778013

Elico-Corp/odoo_OCB,Elico-Corp/odoo_OCB,ygol/odoo,Elico-Corp/odoo_OCB,ygol/odoo,ygol/odoo,Elico-Corp/odoo_OCB,ygol/odoo,ygol/odoo,Elico-Corp/odoo_OCB,ygol/odoo,ygol/odoo,Elico-Corp/odoo_OCB