Remove traversal_ids_btree feature flag

What does this MR do and why?

The <code data-sourcepos="3:7-3:25">traversal_ids_btree</code> FF has been 100% enabled for many months now. This MR will remove the FF.

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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 #369507 (closed)