[FIX] event_sale : unable to delete multiple SO

Step to reproduce:

- go to sale app
- select several SO
- action menu > delete
- got a traceback

Change the unlink definition to get multiple elements with mapped.
This fix an issue introduced on PR 72253

opw-2605699

closes odoo/odoo#73942

Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>

addons/event_sale/models/sale_order.py

@@ -25,7 +25,7 @@ class SaleOrder(models.Model):
         return res
 
     def unlink(self):
-        self.order_line._unlink_associated_registrations()
+        self.mapped('order_line')._unlink_associated_registrations()
         super(SaleOrder, self).unlink()