[Feature flag] Rollout of use_new_zoekt_indexer

What does this MR do and why?

Since this feature flag use_new_zoekt_indexer has been already rolled out globally, this MR is now cleaning up the code related to this FF.

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Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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Related to #414440 (closed)