[FIX] sale, sale_stock: inconsistent decorator

The decorator is inconsistent, and actually useless.

opw-740787

addons/sale/models/sale.py

@@ -442,7 +442,6 @@ class SaleOrder(models.Model):
     def action_done(self):
         self.write({'state': 'done'})
 
-    @api.multi
     def _prepare_procurement_group(self):
         return {'name': self.name}
 

addons/sale_stock/models/sale_order.py

@@ -72,7 +72,6 @@ class SaleOrder(models.Model):
         invoice_vals['incoterms_id'] = self.incoterm.id or False
         return invoice_vals
 
-    @api.model
     def _prepare_procurement_group(self):
         res = super(SaleOrder, self)._prepare_procurement_group()
         res.update({'move_type': self.picking_policy, 'partner_id': self.partner_shipping_id.id})