[MERGE] stock_account: remove property_cost_method and property_valuation

The fields property_cost_method ad property_valuation are never shown on the products but they sometimes are set. This creates confusion and some difficulties in the migration (the cost method/valuation on the products are different than the one set on the categories).

We remove them and only use the one on the categories.

Task: 1866333

closes odoo/odoo#26008