[Feature flag] Remove sync_traversal_ids feature flag

What does this MR do and why?

Remove the sync_traversal_ids feature flag. The Namespace#traversal_ids column will always be kept in sync.

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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• I have evaluated the MR acceptance checklist for this MR.

Closes #321947 (closed)