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

tests/app/dao/test_service_permissions_dao.py

@@ -8,7 +8,6 @@ from app.models import (
     EMAIL_TYPE,
     INBOUND_SMS_TYPE,
     INTERNATIONAL_SMS_TYPE,
-    LETTER_TYPE,
     SMS_TYPE,
 )
 from tests.app.db import create_service, create_service_permission
@@ -29,15 +28,14 @@ def test_create_service_permission(service_without_permissions):
 
 
 def test_fetch_service_permissions_gets_service_permissions(service_without_permissions):
-    create_service_permission(service_id=service_without_permissions.id, permission=LETTER_TYPE)
     create_service_permission(service_id=service_without_permissions.id, permission=INTERNATIONAL_SMS_TYPE)
     create_service_permission(service_id=service_without_permissions.id, permission=SMS_TYPE)
 
     service_permissions = dao_fetch_service_permissions(service_without_permissions.id)
 
-    assert len(service_permissions) == 3
+    assert len(service_permissions) == 2
     assert all(sp.service_id == service_without_permissions.id for sp in service_permissions)
-    assert all(sp.permission in [LETTER_TYPE, INTERNATIONAL_SMS_TYPE, SMS_TYPE] for sp in service_permissions)
+    assert all(sp.permission in [INTERNATIONAL_SMS_TYPE, SMS_TYPE] for sp in service_permissions)
 
 
 def test_remove_service_permission(service_without_permissions):