Remove legacy indexing control code

Background

In !172582 (merged) we're replacing Elastic::IndexingControl with pause control middleware.

Proposal

This is the issue to remove the deprecated code after a required stop.

• %17.9 Remove any code that enqueues the jobs !178182 (merged)
• %17.10 Add a migration that uses sidekiq_remove_jobs !181668 (merged)
• %17.11 Delete the worker class file and related files !185117 (merged)

• Remove code related to dashboards defined in the runbooks repo.

Reference: https://docs.gitlab.com/ee/development/sidekiq/compatibility_across_updates.html#removing-worker-classes