Stop searching the to field

We were doing this temporarily while the `normalised_to` field was not
populated for letters. Once we have a week of data in the
`normalised_to` field we can stop looking in the `to` field. This should
improve performance because:
- it’s one `WHERE` clause not two
- we had to do `ilike` on the `to` field, because we don’t lowercase its
  contents – even if the two where clauses are somehow paralleized it’s
  is slower than a `like` comparison, which is case-sensitive

Depends on:
- [ ] Tuesday 5 May (7 full days after deploying https://github.com/alphagov/notifications-api/pull/2814)

tests/app/dao/notification_dao/test_notification_dao.py

@@ -1319,13 +1319,12 @@ def test_dao_get_notifications_by_reference(
     assert results[1].id == email.id
     assert results[2].id == sms.id
 
-    # If notification_type isn’t specified then we can’t normalise the phone number
-    # to 4477… but this query will still find the 077… variant in the `to` field,
-    # as well as the email
+    # If notification_type isn’t specified then we can’t normalise the
+    # phone number to 4477… so this query will only find the email sent
+    # to 077@example.com
     results = dao_get_notifications_by_recipient_or_reference(service.id, '077')
-    assert len(results) == 2
+    assert len(results) == 1
     assert results[0].id == email.id
-    assert results[1].id == sms.id
 
     results = dao_get_notifications_by_recipient_or_reference(service.id, '077@')
     assert len(results) == 1