libctf: remove outdated comment about parent dict importing

Parent dicts are nowadays imported automatically in most situations, so
the comment in ctf_archive_iter warning people that they need to import
parents by hand is wrong.  Remove it.

libctf/ChangeLog
2021-01-05  Nick Alcock  <nick.alcock@oracle.com>

	* ctf-archive.c (ctf_archive_iter): Remove outdated comment.