Clean up Celery

[microamp]

Celery isn't being used (anymore).

python3.12 -m venv .venv
source .venv/bin/activate
pip install -r requirements/base.txt -r requirements/dev.txt -r requirements/docker.txt

pip freeze | grep -i celery

returns no results.

However, we have some Celery-related environment variables set in ietf/settings/dev.py.

CELERY_EAGER_PROPAGATES_EXCEPTIONS = True
CELERY_ALWAYS_EAGER = True

Some cleanups to follow.