makefs/zfs: Remove a nonsensical comment

When populating files, makefs needs to copy their contents into
userspace in order to compute a checksum, so copy_file_range(2) is out
of the question. Though, it could possibly be used when building other
types of filesystems.

MFC after: 1 week
Sponsored by: The FreeBSD Foundation

(cherry picked from commit 60c95af88f6a63fb7c2c86c6ff9c18743f6bbf0d)