[FIX] point_of_sale: fix memory error

When there are too many (millions) of POS order lines associated to
opened POS sessions we get too many taxes, most are duplicated. This
causes a MemorryError.

Example queries from a real DB:
```
> select count(distinct r.account_tax_id) from pos_order_line l join pos_order o on o.id = l.order_id join pos_session s on s.id = o.session_id join account_tax_pos_order_line_rel r on r.pos_order_
 line_id = l.id where s.state != 'closed'
+-------+
| count |
|-------|
| 24    |
+-------+
> select count(r.account_tax_id) from pos_order_line l join pos_order o on o.id = l.order_id join pos_session s on s.id = o.session_id join account_tax_pos_order_line_rel r on r.pos_order_line_id =
  l.id where s.state != 'closed'
+---------+
| count   |
|---------|
| 2504539 |
+---------+
```

opw-3295467

closes odoo/odoo#124180

X-original-commit: 45d19b265033aa626e110e51d8b1d01408717a3e
Signed-off-by: Christophe Simonis (chs) <chs@odoo.com>