[Feature flag] Remove route_hll_to_snowplow_phase3 FF

What does this MR do and why?

The feature is stable now, the FF can be safely deleted

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

Related to #383657 (closed)