[IMP] base: remove prefetch when getting commercial fields

This is related to issue #31549, fixing it in 10 since it's already a
minor issue here (importing 500 partners which all have the same parent,
on my system the import time goes from 2:30 to 2:00).

This is mostly a problem for such fields as `property_product_pricelist`
(added do the commercial fields list by `product`): since we're first
getting it then writing a field it depends on (updating the address,
which contains the country), the field gets invalidated for all records
on each record being imported, so if we're importing a bunch of partners
which all have the same parent (e.g. company employees)
`property_product_pricelist` is going to be re-computed for every
partner imported so far as well as the one parent we're interested in,
for every new partner we're creating.

The problem is much more prevalent with 12.0's batched creates as we're
first creating all the new partners then doing the updates, and thus for
each new partner we're computing `property_product_pricelist` for all
new partners and the one parent we're interested in, roughly doubling
the import time of a series of partners which all have the same parent.

closes odoo/odoo#31804

Signed-off-by: Raphael Collet (rco) <rco@openerp.com>