Revert "Add maintain_namespace policy"

What does this MR do and why?

This is a revert of !90808 (merged). When I was working on !88414 (merged) I realized that we already have :maintainer_access that fits perfectly to the usage we want to, therefore I'm reverting the policy I added.

Screenshots or screen recordings

no UI change, the details of the usage of this policy can be seen on these MRs: !88414 (diffs) and !92642 (diffs)

How to set up and validate locally

Pipelines should pass

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.