Remove non/crown indicator in letter filenames

This is not required by DVLA and since [1] we no longer care about
the end of letter filenames when collating them, so removing it is
safe to do. Note that the name of the ZIP files of collated letters
is based on a hash of the filenames, which needed updating in tests.

Before merging this we need to do a test run in Staging, so DVLA can
check that a mixture of the old / new filenames won't cause issues.

[1]: https://github.com/alphagov/notifications-api/pull/3172

app/celery/letters_pdf_tasks.py

@@ -62,7 +62,6 @@ def get_pdf_for_templated_letter(self, notification_id):
         notification = get_notification_by_id(notification_id, _raise=True)
         letter_filename = generate_letter_pdf_filename(
             reference=notification.reference,
-            crown=notification.service.crown,
             created_at=notification.created_at,
             ignore_folder=notification.key_type == KEY_TYPE_TEST,
             postage=notification.postage
@@ -373,7 +372,6 @@ def process_sanitised_letter(self, sanitise_data):
         # Now we know if the letter is international, we can check what the filename should be.
         upload_file_name = generate_letter_pdf_filename(
             reference=notification.reference,
-            crown=notification.service.crown,
             created_at=notification.created_at,
             ignore_folder=True,
             postage=notification.postage