Left-over code from optional Celery

__init__.py has left-over code for detecting availability of Celery.

added type::bug label

assigned to @hansegucker

Those people creating issues from live coding sessions ...

added part::backend label

created merge request !773 (merged) to address this issue

mentioned in merge request !773 (merged)

closed with merge request !773 (merged)

mentioned in commit 918140e9