Remove feature flag `ci_refactor_external_rules`

What does this MR do and why?

This MR cleans up the feature flag ci_refactor_external_rules. It was successfully rolled out globally on 2023-08-17 13:00 PT.

This feature was a refactor of the External::Rules class.

• Roll-out issue: #421731 (closed)
• MR that introduced the flag: !129145 (merged)

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MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.

Related to #421731 (closed)