Removes namespace from Sidekiq probe

Sylvester Chinrequested to merge
sc1-remove-dual-poll-function into master

This MR removes the use of namespace in gitlab-exporter's sidekiq probe. It contains 2 commits

  1. reverts commit 02831d8c, reversing changes made to 3ae86e8a.
  2. removes namespace key from the Sidekiq.redis config.

This is part of gitlab-com/gl-infra/scalability#2288

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