Cleanup nuget_duplicates_option feature flag

What does this MR do and why?

In Add a setting for allowing/disallowing duplicat... (!123783 - merged) & Add service to handle disallowing duplicate NuG... (!128269 - merged), the feature flag nuget_duplicates_option was used to gate the disallowing duplicate NuGet package uploads feature.

The feature flag was globally enabled on gitlab.com for nearly 2 months now and was deemed stable. It's time to clean up the feature flag from the code and release the feature.

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Related to #293748 (closed)