Remove sidekiq monitoring web interface

We've changed the approach for doing this to use a standalone
application - https://github.com/alphagov/sidekiq-monitoring. This is
therefore uneeded.

This reverts commit 25d27f19a7db3dd927e1fbb17c7fc6d97804069b.
Merge pull request #258 from alphagov/remove_sidekiq_monitoring

Remove sidekiq monitoring web interface