Remove the use_cte_for_label_filter feature flag

What does this MR do and why?

Remove the use_cte_for_label_filter feature flag.

The label filter CTE optimization fence now applies each time its structural guards hold, with no flag check. The EE override that disables it for epics stays the same.

References

• Feature issue: #603780 (closed)
• Merge request that introduced the flag: !248316 (merged)

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