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/celery/research_mode_tasks.py

@@ -1,13 +1,8 @@
 import json
-import random
-from datetime import datetime, timedelta
 
 from flask import current_app
-from notifications_utils.s3 import s3upload
 from requests import HTTPError, request
 
-from app import notify_celery
-from app.aws.s3 import file_exists
 from app.celery.process_ses_receipts_tasks import process_ses_results
 from app.config import QueueNames
 from app.models import SMS_TYPE
@@ -85,46 +80,6 @@ def sns_callback(notification_id, to):
     #                    "deliverytime": "2016-04-05 16:01:07"})
 
 
-@notify_celery.task(bind=True, name="create-fake-letter-response-file", max_retries=5, default_retry_delay=300)
-def create_fake_letter_response_file(self, reference):
-    now = datetime.utcnow()
-    dvla_response_data = '{}|Sent|0|Sorted'.format(reference)
-
-    # try and find a filename that hasn't been taken yet - from a random time within the last 30 seconds
-    for i in sorted(range(30), key=lambda _: random.random()):  # nosec B311 - not security related
-        upload_file_name = 'NOTIFY-{}-RSP.TXT'.format((now - timedelta(seconds=i)).strftime('%Y%m%d%H%M%S'))
-        if not file_exists(current_app.config['DVLA_RESPONSE_BUCKET_NAME'], upload_file_name):
-            break
-    else:
-        raise ValueError(
-            'cant create fake letter response file for {} - too many files for that time already exist on s3'.format(
-                reference
-            )
-        )
-
-    s3upload(
-        filedata=dvla_response_data,
-        region=current_app.config['AWS_REGION'],
-        bucket_name=current_app.config['DVLA_RESPONSE_BUCKET_NAME'],
-        file_location=upload_file_name
-    )
-    current_app.logger.info("Fake DVLA response file {}, content [{}], uploaded to {}, created at {}".format(
-        upload_file_name, dvla_response_data, current_app.config['DVLA_RESPONSE_BUCKET_NAME'], now))
-
-    # on development we can't trigger SNS callbacks so we need to manually hit the DVLA callback endpoint
-    if current_app.config['NOTIFY_ENVIRONMENT'] == 'development':
-        make_request('letter', 'dvla', _fake_sns_s3_callback(upload_file_name), None)
-
-
-def _fake_sns_s3_callback(filename):
-    message_contents = '{"Records":[{"s3":{"object":{"key":"%s"}}}]}' % (filename)  # noqa
-    return json.dumps({
-        "Type": "Notification",
-        "MessageId": "some-message-id",
-        "Message": message_contents
-    })
-
-
 def ses_notification_callback(reference):
     ses_message_body = {
         'delivery': {