Docs: remove dead code from group-based reward functions snippet

[alexpalms]

Description

Update documentation to remove dead code from group-based reward functions example snippet.

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 would cause existing functionality to not work as expected)
• Documentation update
• Test improvement

Testing

• All existing tests pass when running uv run pytest locally.
• New tests have been added to cover the changes

Checklist

• My code follows the style guidelines of this project as outlined in AGENTS.md
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published

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Note

Low Risk
Low risk documentation-only change that does not affect runtime behavior.

Overview
Cleans up docs/environments.md by removing a dead unique = set(responses) line from the diversity_bonus group-reward example so the snippet no longer includes unused code.

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