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 60c95af88f)

usr.sbin/makefs/zfs/fs.c

@@ -551,8 +551,6 @@ fs_populate_file(fsnode *cur, struct fs_populate_arg *arg)
 
 		/*
 		 * Fill up our buffer, handling partial reads.
-		 *
-		 * It might be profitable to use copy_file_range(2) here.
 		 */
 		sofar = 0;
 		target = MIN(size - foff, (off_t)bufsz);