Revert celery4

Revert the following three pull requests:
https://github.com/alphagov/notifications-api/pull/1085
https://github.com/alphagov/notifications-api/pull/1086
https://github.com/alphagov/notifications-api/pull/1088

celery 4.0.2 looked promising, however, on staging under mild load
(5/sec api calls) the performance was actually worse than 3.1.25

app/celery/research_mode_tasks.py

@@ -1,6 +1,7 @@
 import json
 
 from flask import current_app
+from app import notify_celery
 from requests import request, RequestException, HTTPError
 
 from app.models import SMS_TYPE