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/__init__.py

@@ -1,27 +0,0 @@
-
-class QueueNames(object):
-    PERIODIC = 'periodic-tasks'
-    PRIORITY = 'priority-tasks'
-    DATABASE = 'database-tasks'
-    SEND = 'send-tasks'
-    RESEARCH_MODE = 'research-mode-tasks'
-    STATISTICS = 'statistics-tasks'
-    JOBS = 'job-tasks'
-    RETRY = 'retry-tasks'
-    NOTIFY = 'notify-internal-tasks'
-    PROCESS_FTP = 'process-ftp-tasks'
-
-    @staticmethod
-    def all_queues():
-        return [
-            QueueNames.PRIORITY,
-            QueueNames.PERIODIC,
-            QueueNames.DATABASE,
-            QueueNames.SEND,
-            QueueNames.RESEARCH_MODE,
-            QueueNames.STATISTICS,
-            QueueNames.JOBS,
-            QueueNames.RETRY,
-            QueueNames.NOTIFY,
-            QueueNames.PROCESS_FTP
-        ]