[FIX] purchase: fix notification layout wrongly reset

Commit e38ed7c780 moved code related to purchase, stock and
purchase_stock in order to clean module dependencies. However probably due to
some wrong manipulation notification layout was reset to an old removed
layout. This commit fixes it.

addons/purchase/models/purchase.py

@@ -262,7 +262,7 @@ class PurchaseOrder(models.Model):
             'default_use_template': bool(template_id),
             'default_template_id': template_id,
             'default_composition_mode': 'comment',
-            'custom_layout': "purchase.mail_template_data_notification_email_purchase_order",
+            'custom_layout': "mail.mail_notification_borders",
             'force_email': True,
             'mark_rfq_as_sent': True,
         })