Clean up default_to_import_tab experiment

What does this MR do and why?

This issue is to cleanup and remove the default_to_import_tab feature flag as the experiment was deemed experimentinvalidated (summary)

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

How to set up and validate locally

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Related to #452207