🧹 Remove unused _review_key and sorting in mapping reviewer calibration

[edithatogo]

🎯 What: Removed the unused _review_key function and the sorting calls that depended on it in build_mapping_reviewer_calibration.py. Fixed ambiguity lint warnings in _agreement_metrics.
💡 Why: The function was identified as potentially no longer needed for sorting, and its removal improves maintainability by removing unnecessary sorting complexity and dead code. Also fixed E741 linter errors for ambiguous variables.
✅ Verification: Verified by confirming the related test suite (test_mapping_reviewer_calibration.py) passes and there are no regressions. Ran ruff check and confirmed formatting is clean.
✨ Result: The code is cleaner, more readable, and adheres better to Python conventions.

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PR created automatically by Jules for task 3600745046709291720 started by @edithatogo

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