Remove the upload letters permission

As of https://github.com/alphagov/notifications-admin/pull/3690 it’s no
longer referred to.

app/dao/services_dao.py

@@ -50,7 +50,6 @@ from app.models import (
     NOTIFICATION_PERMANENT_FAILURE,
     SMS_TYPE,
     LETTER_TYPE,
-    UPLOAD_LETTERS,
     INTERNATIONAL_LETTERS
 )
 from app.utils import (
@@ -66,7 +65,6 @@ DEFAULT_SERVICE_PERMISSIONS = [
     EMAIL_TYPE,
     LETTER_TYPE,
     INTERNATIONAL_SMS_TYPE,
-    UPLOAD_LETTERS,
     INTERNATIONAL_LETTERS,
 ]