add tests for letters not getting excluded

app/dao/notifications_dao.py

@@ -346,13 +346,10 @@ def _filter_query(query, filter_dict=None):
 @statsd(namespace="dao")
 def delete_notifications_created_more_than_a_week_ago(status):
     seven_days_ago = date.today() - timedelta(days=7)
-
     deleted = db.session.query(Notification).filter(
         func.date(Notification.created_at) < seven_days_ago,
         Notification.status == status,
-        Notification.notification_type != LETTER_TYPE
     ).delete(synchronize_session='fetch')
-
     db.session.commit()
     return deleted