chore(expr): cleanup code for kernels/group_by

[andylokandy]

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Summary

Briefly describe what this PR aims to solve. Include background context that will help reviewers understand the purpose of the PR.

Fixes #[Link the issue here]

Tests

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• No Test - Explain why

Type of change

• Bug Fix (non-breaking change which fixes an issue)
• New Feature (non-breaking change which adds functionality)
• Breaking Change (fix or feature that could cause existing functionality not to work as expected)
• Documentation Update
• Refactoring
• Performance Improvement
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This change is