Remove letters-related code (#175)

This deletes a big ol' chunk of code related to letters. It's not everything—there are still a few things that might be tied to sms/email—but it's the the heart of letters function. SMS and email function should be untouched by this.

Areas affected:

- Things obviously about letters
- PDF tasks, used for precompiling letters
- Virus scanning, used for those PDFs
- FTP, used to send letters to the printer
- Postage stuff

tests/app/notifications/test_rest.py

@@ -13,20 +13,17 @@ from tests import create_service_authorization_header
 from tests.app.db import create_api_key, create_notification
 
 
-@pytest.mark.parametrize('type', ('email', 'sms', 'letter'))
+@pytest.mark.parametrize('type', ('email', 'sms'))
 def test_get_notification_by_id(
     client,
     sample_notification,
     sample_email_notification,
-    sample_letter_notification,
     type
 ):
     if type == 'email':
         notification_to_get = sample_email_notification
     if type == 'sms':
         notification_to_get = sample_notification
-    if type == 'letter':
-        notification_to_get = sample_letter_notification
 
     auth_header = create_service_authorization_header(service_id=notification_to_get.service_id)
     response = client.get(