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/v2/templates/test_get_templates.py

@@ -112,7 +112,7 @@ def test_get_all_templates_for_invalid_type_returns_400(client, sample_service):
         'status_code': 400,
         'errors': [
             {
-                'message': 'type coconut is not one of [sms, email, letter]',
+                'message': 'type coconut is not one of [sms, email]',
                 'error': 'ValidationError'
             }
         ]

tests/app/v2/templates/test_templates_schemas.py

@@ -241,7 +241,7 @@ def test_get_all_template_request_schema_against_invalid_args_is_invalid(templat
 
     assert errors['status_code'] == 400
     assert len(errors['errors']) == 1
-    assert errors['errors'][0]['message'] == 'type unknown is not one of [sms, email, letter]'
+    assert errors['errors'][0]['message'] == 'type unknown is not one of [sms, email]'
 
 
 @pytest.mark.parametrize("response", valid_json_get_all_response)