Cleanup unchanged overridden method _extract_variable_selector_to_variable_mapping in subclass of BaseNode

[bowenliang123]

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1. Is this request related to a challenge you're experiencing? Tell me about your story.

The subclaess of BaseNode 's overridden method _extract_variable_selector_to_variable_mapping is identical to its parent class BaseNode , which can be removed.

As the follow-up PR with #14174.

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