Remove documentation for 'feature_state_counters'

[germain-gg]

For #25884

Checklist

• Tests written for new code (and old code if feasible)
• Linter and other CI checks pass
• Sign-off given on the changes (see CONTRIBUTING.md)

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This change is marked as an internal change (Task), so will not be included in the changelog.

[Johennes]

Reopening this because this feature appears to be available with both the old and new header. So removing it doesn't have to block on removing the old header code.