Stop killing reporting processes after each task

Previously we think this setting was necessary to avoid a memory
leak [1], but it's unclear if this is still an issue:

- We've advanced two major versions of Celery.
- Some of the tasks are now quicker and leaner.

Restarting worker sub-processes after each task is a big problem
for performance, as we move towards parallelising our reporting.

This is something of a test to see if we can manage without this
setting. Note that we need to unset the variable manually:

   cf unset-env notify-delivery-worker-reporting CELERYD_MAX_TASKS_PER_CHILD

In the worst case we can always re-run any failed tasks. To check
the worker is still behaving as expected, we can:

- Monitor CPU / memory graphs for it.
- Check `cf events` for unexpected restarts / crashes.
- Compare numbers of task completion logs to previous days.
- Check the number of new billing / status rows looks right.

[1]: https://github.com/alphagov/notifications-api/commit/ad419f7592524292f8da8447814cc0bbc37f240e

manifest.yml.j2

@@ -56,7 +56,6 @@
   'notify-delivery-worker-periodic': {},
   'notify-delivery-worker-reporting': {
     'additional_env_vars': {
-      'CELERYD_MAX_TASKS_PER_CHILD': 1,
       'CELERYD_PREFETCH_MULTIPLIER': 1,
       'SQLALCHEMY_STATEMENT_TIMEOUT': 7200
     }