Reverts 891a1d0b, because the new  psql command \dX only worked for
users users who can read pg_statistic_ext_data catalog, and most regular
users lack that privilege (the catalog may contain sensitive user data).

Reported-by: Noriyoshi Shinoda
Discussion: https://postgr.es/m/c027a541-5856-75a5-0868-341301e1624b%40nttcom.co.jp_1