[FIX] pos_restaurant: correctly save splitted order

Current behavior:
When splitting the order in a pos_restaurant session, the order was not
correctly saved in the backend. It resulted in an order that still had
all the product of the original order and another order with the
correctly splitted products.

Steps to reproduce:
- Start a pos_restaurant session.
- Go to table A and add 3 products to the table.
- Go back to the floor screen
- Go back to the table A
- Split the order in 2
- Click on payment, and go back to the product screen without paying
- Click on the ticket button, and click on the order that was splitted,
  it still contains all the products from the original order.

opw-3061569

closes odoo/odoo#106553

X-original-commit: a61c2b88
Signed-off-by: Trinh Jacky (trj) <trj@odoo.com>