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/serialised_models.py

@@ -41,7 +41,6 @@ class SerialisedTemplate(SerialisedModel):
         'archived',
         'content',
         'id',
-        'postage',
         'process_type',
         'reply_to_text',
         'subject',