Delete CODEOWNERS

Created by: nicksnyder

I think CODEOWNERS is broken. I see PRs where tons of reviewers get added, and I see PRs where the wrong reviewers get added. It isn't expressive enough to actually capture the right set of reviewers. Worse, it is unclear whose approvals are needed to actually merge. Without CODEOWNERS, GitHub will suggest reviewers but won't automatically add them. This puts the responsibility on the author to determine who is the best reviewers (e.g., who wrote the file? who last edited the file? who last reviewed a change to this file), which I think is a good thing.

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