Should we remove db_user_namespace?

I think this is the second decennial thread [0] for removing this GUC.
This topic came up at PGCon, so I thought I'd start the discussion on the
lists.

I'm personally not aware of anyone using this parameter. A couple of my
colleagues claimed to have used it in the aughts, but they also noted that
users were confused by the current implementation, and they seemed
generally in favor of removing it. AFAICT the strongest reason for keeping
it is that there is presently no viable replacement. Does this opinion
still stand? If so, perhaps we can look into adding a viable replacement
for v17.

The attached patch simply removes the GUC.

[0] https://postgr.es/m/CAA-aLv6wnwp6Qr5fZo%2B7K%3DVSr51qFMuLsCeYvTkSF3E5qEPvqA%40mail.gmail.com

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Nathan Bossart
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