refactor: remove dead code and redundant checks

[Galoretka]

• Delete unreachable gate nil-checks and the if gate != nil wrapper in SolveInTestEngine; gkrgates.Get panics on missing gates, so these branches could never execute.
• Remove the no-op continue in the preparatory loop and drop the unused loop variable.

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Note

Streamlines SolveInTestEngine by removing dead gate nil-checks/conditional evaluation and simplifying the circuit initialization loop.

• gkrapi:
  • std/gkrapi/testing.go (SolveInTestEngine):
    • Remove redundant gate == nil checks and the if gate != nil wrapper; always evaluate gate after retrieval and verification.
    • Simplify initialization loop over api.toStore.Circuit: drop unused loop variable and the no-op continue.

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