Enable the `use_unnested_queries` by removing the FF globally

Mehmet Emin INACrequested to merge
366818_cleanup_use_unnested_queries_ff into master

What does this MR do and why?

Cleanup the use_unnested_queries FF to enable it globally.

Related to [Feature flag] Enable `use_unnested_queries` (#366818 - closed).

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

Edited by Mehmet Emin INAC

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