Disallow comments on columns of relation types other than tables, views,

Disallow comments on columns of relation types other than tables, views,
and composite types, which are the only relkinds for which pg_dump support
exists for dumping column comments.  There is no obvious usefulness for
comments on columns of sequences or toast tables; and while comments on
index columns might have some value, it's not worth the risk of compatibility
problems due to possible changes in the algorithm for assigning names to
index columns.  Per discussion.

In consequence, remove now-dead code for copying such comments in CREATE TABLE
LIKE.