pgsql: Update obsolete comment in index_drop().

Log Message:
-----------
Update obsolete comment in index_drop(). When the comment was written,
queries frequently took no lock at all on individual indexes. That's not
true any more, but we still need lock on the parent table to make it safe
to use cached lists of index OIDs.

Modified Files:
--------------
pgsql/src/backend/catalog:
index.c (r1.315 -> r1.316)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/catalog/index.c?r1=1.315&r2=1.316)