Revert "Merge branch 'osw-use-sidekiq-cluster-by-default' into 'master'"

What does this Merge Request do?

It reverts !1120 (merged).

I've noticed it's timing out when trying to start sidekiq-cluster. I'd rather revert it for now to understand the issue a bit better:

› gdk start rails-background-jobs
(in /Users/osw/projects/gitlab/gdk-ee)
timeout: down: ./services/rails-background-jobs: 1s, want up