Remove out-of-date comment about formrdesc().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but the list was out of date.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

src/backend/utils/cache/relcache.c

@@ -1847,11 +1847,7 @@ LookupOpclassInfo(Oid operatorClassOid,
  *		quite a lot since we only need to work for a few basic system
  *		catalogs.
  *
- * formrdesc is currently used for: pg_database, pg_authid, pg_auth_members,
- * pg_shseclabel, pg_class, pg_attribute, pg_proc, and pg_type
- * (see RelationCacheInitializePhase2/3).
- *
- * Note that these catalogs can't have constraints (except attnotnull),
+ * The catalogs this is used for can't have constraints (except attnotnull),
  * default values, rules, or triggers, since we don't cope with any of that.
  * (Well, actually, this only matters for properties that need to be valid
  * during bootstrap or before RelationCacheInitializePhase3 runs, and none of