Remove `_close_db_handle` method from `QueueWorker` class. Nowhere
calls this method, so it is safe to remove. This function was the
only place using the `db` imported from `data.model`, so we can
remove that import as well.
Testing: need to look into it
there should be two lines between functions and other code
Issue: https://www.pivotaltracker.com/story/show/b144646649n
- [ ] It works!
- [ ] Comments provide sufficient explanations for the next contributor
- [ ] Tests cover changes and corner cases
- [ ] Follows Quay syntax patterns and format
