[FIX] point_of_sale: remove monetary widget of discount field

Considering the field declaration:
https://github.com/odoo/odoo/blob/fe5deb4ee4c8e07aed2f2cff6210271bfaa61476/addons/point_of_sale/models/pos_order.py#L1121


`discount` is a rate, not an amount. Therefore, suppose a discount of
10%, when opening a POS-generated SO, the discount shouldn't be "$10".

OPW-2629493

closes odoo/odoo#75829

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