[IMP] sale_management: remove price_unit from quotation template

Purpose
=======

Having price and discount on quotation template is quite confusing
as it is more the job of pricelists.
In order to make it more simple for the user, we should remove the
fields price_unit and discount from quotation template.

Implementation
========

Now, the public price of a product is used instead of the price from
the quotation template. If a pricelist exists, it is taken into
account as before.

+ Add new tests to check the sale order template behavior.

closes odoo/odoo#47186

Taskid: 47186
Related: odoo/upgrade#972
Signed-off-by: Yannick Tivisse (yti) <yti@odoo.com>