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_senders_schema.py

@@ -13,19 +13,6 @@ add_service_email_reply_to_request = {
 }
 
 
-add_service_letter_contact_block_request = {
-    "$schema": "http://json-schema.org/draft-07/schema#",
-    "description": "POST service letter contact block",
-    "type": "object",
-    "title": "Add new letter contact block for service",
-    "properties": {
-        "contact_block": {"type": "string"},
-        "is_default": {"type": "boolean"}
-    },
-    "required": ["contact_block", "is_default"]
-}
-
-
 add_service_sms_sender_request = {
     "$schema": "http://json-schema.org/draft-07/schema#",
     "description": "POST add service SMS sender",