Remove the deprected Zoekt::IndexerWorker

What does this MR do and why?

Remove the worker Zoekt::IndexerWorker and its occurrences. Since all the zoekt indexing is done via tasks we don't use this indexer anymore. Since we are using tasks for indexing we don't need to use _in. This delay was added first to reduce the load on Sidekiq. With tasks we don't use sidekiq.

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Related to #496749 (closed)