Remove example Sidekiq config

This is nice to have, but the current version has a few problems:

1. As it doesn't use negateQueues anywhere, it won't pick up any new queues added on the application side without manual changes.
2. it isn't compatible with the Sidekiq Cluster method of selecting queues, which will become the default in 13.0.
3. It's not really clear how the workload is picked - in particular, the ee pod seems to run queues based on the edition of GitLab they're in, not based on their workloads.

In gitlab-com/gl-infra&174 (closed) we're making it possible to route queues in a different way, as a step towards gitlab-com/gl-infra&194 (closed) where we'd use many fewer queues.

Closes #1979 (closed).