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/performance_platform/total_sent_notifications.py

@@ -4,6 +4,7 @@ from app.dao.fact_notification_status_dao import (
 )
 
 
+# TODO: is this obsolete? it doesn't seem to be used anywhere
 def send_total_notifications_sent_for_day_stats(start_time, notification_type, count):
     payload = performance_platform_client.format_payload(
         dataset='notifications',
@@ -16,13 +17,12 @@ def send_total_notifications_sent_for_day_stats(start_time, notification_type, c
     performance_platform_client.send_stats_to_performance_platform(payload)
 
 
+# TODO: is this obsolete? it doesn't seem to be used anywhere
 def get_total_sent_notifications_for_day(day):
     email_count = get_total_sent_notifications_for_day_and_type(day, 'email')
     sms_count = get_total_sent_notifications_for_day_and_type(day, 'sms')
-    letter_count = get_total_sent_notifications_for_day_and_type(day, 'letter')
 
     return {
         "email": email_count,
         "sms": sms_count,
-        "letter": letter_count,
     }