[FIX] website_crm_partner_assign: onchange_assign_id new API conversion oversight

You are not supposed to write anything in an onchange,
the result of the onchange must be assigned in the given fields
of `self`

This is an oversight during the migration to the new api of the module
in the below revision:
053982f4

opw-694053

addons/website_crm_partner_assign/models/crm_lead.py

@@ -33,10 +33,8 @@ class CrmLead(models.Model):
         if not partner_assigned:
             self.date_assign = False
         else:
-            self.write({
-                'date_assign': fields.Date.context_today(self),
-                'user_id': partner_assigned.user_id,
-            })
+            self.date_assign = fields.Date.context_today(self)
+            self.user_id = partner_assigned.user_id
 
     @api.multi
     def assign_salesman_of_assigned_partner(self):