[FIX] account: Wrong unit price with included tax and fiscal position

Steps to reproduce the bug:

- Let's consider a sale included tax T1 of 10% and a sale excluded tax T2 of 0%
- Let's consider a product P with T1 and a sale price of 110€
- Let's consider a fiscal position FP that mappes T1 to T2
- Let's consider a customer C with FP as fiscal position
- Create a customer invoice for C
- Add P on the first line and T1 is replaced by T2

Bug:

The unit price of P was still 110€ instead of 100€ because the included tax was not removed from the base
price of P.

Same behavior as in 11.0 and 12.0

opw:2150564

closes odoo/odoo#42188

Signed-off-by: oco-odoo <oco-odoo@users.noreply.github.com>