keep built-in default value for notify_email field,

because im_and_email value can bring a problems after uninstalling the module.

im_notif/im_notif_models.py

@@ -19,10 +19,6 @@ class res_partner(models.Model):
                                      ),
     }
 
-    _defaults = {
-        'notify_email': lambda *args: 'im_and_email'
-    }
-
 class mail_notification(models.Model):
     _inherit = 'mail.notification'