Cleanup UserNamespace TODO code

What does this MR do and why?

Second bit of #341070 (closed)

Search the code for https://gitlab.com/gitlab-org/gitlab/-/issues/341070 for TODOs that indicate how code should be updated regarding UserNamespaces.

This MR is for cleaning up code used to facilitate migrating all type == nil namespaces to proper UserNamespaces

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Related to #341070 (closed)