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

app/service/service_data_retention_schema.py

@@ -5,7 +5,7 @@ add_service_data_retention_request = {
     "type": "object",
     "properties": {
         "days_of_retention": {"type": "integer"},
-        "notification_type": {"enum": ["sms", "letter", "email"]},
+        "notification_type": {"enum": ["sms", "email"]},
     },
     "required": ["days_of_retention", "notification_type"]
 }