pgsql: Remove db_user_namespace.

Remove db_user_namespace.

This feature was intended to be a temporary measure to support
per-database user names. A better one hasn't materialized in the
~21 years since it was added, and nobody claims to be using it, so
let's just remove it.

Reviewed-by: Michael Paquier, Magnus Hagander
Discussion: https://postgr.es/m/20230630200509.GA2830328%40nathanxps13
Discussion: https://postgr.es/m/20230630215608.GD2941194%40nathanxps13

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/884eee5bfb5ab902413dc00ccee5b3e3f1c505d0

Modified Files
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doc/src/sgml/client-auth.sgml | 5 ---
doc/src/sgml/config.sgml | 52 ---------------------------
src/backend/libpq/auth.c | 5 ---
src/backend/libpq/hba.c | 12 -------
src/backend/postmaster/postmaster.c | 19 ----------
src/backend/utils/misc/guc_tables.c | 9 -----
src/backend/utils/misc/postgresql.conf.sample | 1 -
src/include/libpq/pqcomm.h | 2 --
8 files changed, 105 deletions(-)