t/lib-submodule-update: remove outdated test description
The commands in the unpack_trees machinery (checkout, reset, read-tree)
were fixed in
218c883783 (submodule: properly recurse for read-tree and
checkout, 2017-05-02) to correctly update nested submodules when called
with the `--recurse-submodules` flag.
However, a comment in t/lib-submodule-update.sh mentions that this use
case still doesn't work.
Remove this outdated comment.
Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>