docs: update outdated Celery beat documentation

[nijel]

Proposed changes

There is no longer a separate database that could be corrupt, it uses same database as Weblate.

Checklist

• Lint and unit tests pass locally with my changes.
• I have added tests that prove my fix is effective or that my feature works.
• I have added documentation to describe my feature.
• I have squashed my commits into logic units.
• I have described the changes in the commit messages.

Other information