[FIX] point_of_sale: resuscitate onchange amount all
[FIX] point_of_sale: resuscitate onchange amount all

Make a return on a pos_order. A new pos_order is created.
On the latter, remove a pos_orde_line.

Before this commit, there was a traceback, due to a KeyError
It was introduced by 5f6a4912659084af16856fac164e175acfd712f4
which intended to optimize migration process

We keep the logic of that commit here, in a new helper method
while restoring the original onchange,
since it wasn't compliant with onchange specs

OPW 1934129

closes odoo/odoo#30897
ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo