[IMP] account: improve discount fields in move view

As of now, the discount field is either only shown in
debug mode or if the sale module is installed and a
setting is set in sale.
This field should be shown in the view, and not depend
on a setting from another app.
This is especially true as it could be filled by the ocr
or when uploading from an edi, and it would be inconsistent
if the user couldn't display it.

Task id #2758965

closes odoo/odoo#85652

X-original-commit: 37feb941
Signed-off-by: Josse Colpaert <jco@odoo.com>
Signed-off-by: Nicolas Viseur <vin@odoo.com>