unused, undocumented method causing errors on rubydoc.info

Sidekiq's documentation is [not available on rubydoc.info](http://rubydoc.info/gems/sidekiq/Sidekiq) due to a method in `::Sidekiq` which does not appear to be part of the public interface, and is not called from anywhere inside the gem.

This pull request removes the dead code, which restores the functionality of the documentation server. Tested against `yard server` v0.8.7.2 and ruby 2.0.0-p247.