Revert "Merge branch '335378-fix-code-owners-section' into 'master'"

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Fixes #346616 (closed)

The EE-only constant Gitlab::CodeOwners is being referred to by a core class.

Reverts !74298 (merged)

It seems the whole MR needs to be moved to ee/, so unsure of any trivial fixes. So reverting is best for now

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