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/v2/notifications/get_notifications.py

@@ -1,18 +1,8 @@
-from io import BytesIO
-
-from flask import current_app, jsonify, request, send_file, url_for
+from flask import current_app, jsonify, request, url_for
 
 from app import api_user, authenticated_service
 from app.dao import notifications_dao
-from app.letters.utils import get_letter_pdf_and_metadata
-from app.models import (
-    LETTER_TYPE,
-    NOTIFICATION_PENDING_VIRUS_CHECK,
-    NOTIFICATION_TECHNICAL_FAILURE,
-    NOTIFICATION_VIRUS_SCAN_FAILED,
-)
 from app.schema_validation import validate
-from app.v2.errors import BadRequestError, PDFNotReadyError
 from app.v2.notifications import v2_notification_blueprint
 from app.v2.notifications.notification_schemas import (
     get_notifications_request,
@@ -30,34 +20,6 @@ def get_notification_by_id(notification_id):
     return jsonify(notification.serialize()), 200
 
 
-@v2_notification_blueprint.route('/<notification_id>/pdf', methods=['GET'])
-def get_pdf_for_notification(notification_id):
-    _data = {"notification_id": notification_id}
-    validate(_data, notification_by_id)
-    notification = notifications_dao.get_notification_by_id(
-        notification_id, authenticated_service.id, _raise=True
-    )
-
-    if notification.notification_type != LETTER_TYPE:
-        raise BadRequestError(message="Notification is not a letter")
-
-    if notification.status == NOTIFICATION_VIRUS_SCAN_FAILED:
-        raise BadRequestError(message='File did not pass the virus scan')
-
-    if notification.status == NOTIFICATION_TECHNICAL_FAILURE:
-        raise BadRequestError(message='PDF not available for letters in status {}'.format(notification.status))
-
-    if notification.status == NOTIFICATION_PENDING_VIRUS_CHECK:
-        raise PDFNotReadyError()
-
-    try:
-        pdf_data, metadata = get_letter_pdf_and_metadata(notification)
-    except Exception:
-        raise PDFNotReadyError()
-
-    return send_file(path_or_file=BytesIO(pdf_data), mimetype='application/pdf')
-
-
 @v2_notification_blueprint.route("", methods=['GET'])
 def get_notifications():
     _data = request.args.to_dict(flat=False)