Remove deprecated Sidekiq rake tasks

It doesn't look like these have been used since GitLab 6.0!

    $ git describe 6a7d63aa
    v6.0.0-575-g6a7d63aa

changelogs/unreleased/remove-sidekiq-rake-tasks.yml

+---
+title: Remove deprecated Sidekiq rake tasks
+merge_request:
+author:
+type: removed

lib/tasks/sidekiq.rake

-namespace :sidekiq do
-  def deprecation_warning!
-    warn <<~WARNING
-      This task is deprecated and will be removed in 13.0 as it is thought to be unused.
-
-      If you are using this task, please comment on the below issue:
-        https://gitlab.com/gitlab-org/gitlab/issues/196731
-    WARNING
-  end
-
-  desc '[DEPRECATED] GitLab | Sidekiq | Stop sidekiq'
-  task :stop do
-    deprecation_warning!
-
-    system(*%w(bin/background_jobs stop))
-  end
-
-  desc '[DEPRECATED] GitLab | Sidekiq | Start sidekiq'
-  task :start do
-    deprecation_warning!
-
-    system(*%w(bin/background_jobs start))
-  end
-
-  desc '[DEPRECATED] GitLab | Sidekiq | Restart sidekiq'
-  task :restart do
-    deprecation_warning!
-
-    system(*%w(bin/background_jobs restart))
-  end
-
-  desc '[DEPRECATED] GitLab | Sidekiq | Start sidekiq with launchd on Mac OS X'
-  task :launchd do
-    deprecation_warning!
-
-    system(*%w(bin/background_jobs start_silent))
-  end
-end