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

migrations/versions/0370_remove_reach.py

@@ -12,8 +12,6 @@ from datetime import datetime
 from alembic import op
 from sqlalchemy.sql import text
 
-from app.models import LetterRate
-
 
 revision = '0370_remove_reach'
 down_revision = '0369_update_sms_rates'