mkfs.xfs: remove comment about needed future work

mkfs.xfs: remove comment about needed future work

Remove comment about the need to sync this function with the
kernel; that was mostly taken care of with:

7b7548052 ("mkfs: use libxfs to write out new AGs")

There's maybe a little more samey-samey that we could do here,
but it's not egregiously cut & pasted as it was before.

Fixes: 7b7548052d12 ("mkfs: use libxfs to write out new AGs")
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>