[REF] point_of_sale: remove stock location from POS config

There is currently a stock location that is set on the pos config which
is by default the the default source of the picking type. This field is
not really usefull and we prefer that the configuration of the source
location where the POS will take products come from a dedicated
picking type that will have the correct default source location.

So we have removed the stock_location_id from the pos_config and changed
the related set on orders, by the location of the picking set on the
pos order, as it was not really consistent that if the pos_config was
changed the location of all existing orders was changed too.

TASK-ID: 1970513

closes odoo/odoo#32870

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