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/template_statistics/rest.py

@@ -37,7 +37,6 @@ def get_template_statistics_for_service_by_day(service_id):
             'template_id': str(row.template_id),
             'template_name': row.template_name,
             'template_type': row.notification_type,
-            'is_precompiled_letter': row.is_precompiled_letter,
             'status': row.status
         }
         for row in data