Clean up `package_registry_access_level` feature flag

What does this MR do and why?

The package_registry_access_level feature flag was introduced in !82808 (merged) and was used for the implementation of #329253 (comment 907634930). The feature flag will be rolled out in #363018 (closed).

This MR cleans up the package_registry_access_level feature flag.

Cleanup issue: #385994 (closed).

🛠 with ❤ at Siemens

/cc @bufferoverflow

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