perf: no caching for prefer timezone

`Prefer: timezone` no longer requires the schema cache.
Previously this required caching `pg_timezone_names` which was slow in some systems.

Closes https://github.com/PostgREST/postgrest/issues/5100 and
https://github.com/PostgREST/postgrest/issues/4751.

docs/references/admin_server.rst

@@ -74,5 +74,4 @@ Provides the ``schema_cache`` endpoint that prints the runtime :ref:`schema_cach
     "dbRepresentations": ["..."],
     "dbRoutines": ["..."],
     "dbTables": ["..."],
-    "dbTimezones": ["..."]
   }