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/schema_validation/__init__.py

@@ -35,14 +35,6 @@ def validate_schema_email_address(instance):
     return True
 
 
-@format_checker.checks('postage', raises=ValidationError)
-def validate_schema_postage(instance):
-    if isinstance(instance, str):
-        if instance not in ["first", "second", "europe", "rest-of-world"]:
-            raise ValidationError("invalid. It must be first, second, europe or rest-of-world.")
-    return True
-
-
 @format_checker.checks('datetime_within_next_day', raises=ValidationError)
 def validate_schema_date_with_hour(instance):
     if isinstance(instance, str):