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/dao/permissions_dao.py

@@ -6,7 +6,6 @@ from app.models import (
     MANAGE_TEMPLATES,
     MANAGE_USERS,
     SEND_EMAILS,
-    SEND_LETTERS,
     SEND_TEXTS,
     VIEW_ACTIVITY,
     Permission,
@@ -19,7 +18,6 @@ default_service_permissions = [
     MANAGE_SETTINGS,
     SEND_TEXTS,
     SEND_EMAILS,
-    SEND_LETTERS,
     MANAGE_API_KEYS,
     VIEW_ACTIVITY]