Revert "[FIX] purchase: update order_lines date_planned"

This reverts commit dcf50bf242.

The commit should not have been forward-ported after 9.0 since
Model update method has been @api.multi'ed.

This also introduced an issue since changing the first line would
change the date_planned of the order (which would alter subesquent
updates).

opw-697680

addons/purchase/purchase.py

@@ -479,9 +479,7 @@ class PurchaseOrder(models.Model):
     @api.multi
     def action_set_date_planned(self):
         for order in self:
-            #DO NOT FORWARD PORT
-            for line in order.order_line:
-                line.update({'date_planned': order.date_planned})
+            order.order_line.update({'date_planned': order.date_planned})
 
 
 class PurchaseOrderLine(models.Model):