Remove the delete query when updating the ft_billing. It's in the wrong place and we also should not need it.

app/dao/fact_billing_dao.py

@@ -159,7 +159,7 @@ def fetch_billing_data_for_day(process_day, service_id=None):
                 notification_type=notification_type,
                 start_date=start_date,
                 end_date=end_date,
-                service_id = id_of_service
+                service_id=id_of_service
             )
             # If data has been purged from Notification then use NotificationHistory
             if len(results) == 0:
@@ -285,10 +285,6 @@ def update_fact_billing(data, process_day):
                     data.postage)
     billing_record = create_billing_record(data, rate, process_day)
 
-    FactBilling.query.filter(
-        FactBilling.bst_date == process_day
-    ).delete()
-
     table = FactBilling.__table__
     '''
        This uses the Postgres upsert to avoid race conditions when two threads try to insert