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/test_config.py

@@ -4,7 +4,7 @@ from app.config import QueueNames
 def test_queue_names_all_queues_correct():
     # Need to ensure that all_queues() only returns queue names used in API
     queues = QueueNames.all_queues()
-    assert len(queues) == 17
+    assert len(queues) == 15
     assert set([
         QueueNames.PRIORITY,
         QueueNames.PERIODIC,
@@ -16,10 +16,8 @@ def test_queue_names_all_queues_correct():
         QueueNames.JOBS,
         QueueNames.RETRY,
         QueueNames.NOTIFY,
-        QueueNames.CREATE_LETTERS_PDF,
         QueueNames.CALLBACKS,
         QueueNames.CALLBACKS_RETRY,
-        QueueNames.LETTERS,
         QueueNames.SMS_CALLBACKS,
         QueueNames.SAVE_API_EMAIL,
         QueueNames.SAVE_API_SMS,