Cost savings

The "cost" value was flawed for a couple of reasons.

1. Lots of messages are free, so in those instances the "cost"
   doesn't tell you anything
2. The query to get the rate was expensive and we don't have
   an obvious way to get it back very efficiently for large numbers
   of notifications.

So we scrapped it.

tests/app/v2/notifications/test_get_notifications.py

@@ -46,7 +46,6 @@ def test_get_notification_by_id_returns_200(
         'line_5': None,
         'line_6': None,
         'postcode': None,
-        'cost': sample_notification.cost(),
         'type': '{}'.format(sample_notification.notification_type),
         'status': '{}'.format(sample_notification.status),
         'template': expected_template_response,