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/tasks.py

@@ -19,8 +19,8 @@ from app import (
 )
 from app.aws import s3
 from app.celery import provider_tasks
+from app.config import QueueNames
 from app.dao.inbound_sms_dao import dao_get_inbound_sms_by_id
-from app.celery import QueueNames
 from app.dao.jobs_dao import (
     dao_update_job,
     dao_get_job_by_id,