Update a code comment

Update a code comment

The comment explained that ALTER TABLE ADD CONSTRAINT USING INDEX is
only supported with a btree index.  (This is not being changed.)  The
reason is to keep upgrades robust, as explained there.  The other part
of the comment, that btree is the only unique index kind anyway, is
somewhat less true as we're trying to enable unique indexes other than
btree, and it's irrelevant to this check.  There is a check for
indisunique earlier already.  So just remove this part of the comment.

Author: Mark Dilger <mark.dilger@enterprisedb.com>
Discussion: https://www.postgresql.org/message-id/flat/E72EAA49-354D-4C2E-8EB9-255197F55330@enterprisedb.com