Remove out-of-date comment about formrdesc().

Remove out-of-date comment about formrdesc().

formrdesc's comment listed the specific catalogs it is called for,
but the list was out of date.  Rather than jumping back onto that
maintenance treadmill, let's just remove the list.  It tells the
reader nothing that can't be learned quickly and more reliably by
searching relcache.c for callers of formrdesc().

Oversight noted by Kyotaro Horiguchi.

Discussion: https://postgr.es/m/20180214.105314.138966434.horiguchi.kyotaro@lab.ntt.co.jp