refactor: remove unused integrated gate name helper

[kurahin]

The integrated gate name helper on roundGateNamer was never used anywhere in the Poseidon2 GKR circuit or in gate registration. Removed the method and its comment.

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[!NOTE] Remove the unused roundGateNamer.integrated gate name helper from std/permutation/poseidon2/gkr-poseidon2/gkr.go.

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