Initial submitted versions of the functional dependencies patch ignored
row groups that were smaller than a configured size.  However, that
consideration was removed in late stages of the patch just before
commit, but some comments referring to it remained.  Remove them to
avoid confusion.

Author: Atsushi Torikoshi
Discussion: https://postgr.es/m/7cfb23fc-4493-9c02-5da9-e505fd0115d2@lab.ntt.co.jp