t/lib-submodule-update: remove outdated test description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Remove this outdated comment.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>
diff --git a/t/lib-submodule-update.sh b/t/lib-submodule-update.sh
index 1dd17fc..5f9eb68 100755
--- a/t/lib-submodule-update.sh
+++ b/t/lib-submodule-update.sh
@@ -908,7 +908,6 @@
 		)
 	'
 
-	# recursing deeper than one level doesn't work yet.
 	test_expect_success "$command: modified submodule updates submodule recursively" '
 		prolog &&
 		reset_work_tree_to_interested add_nested_sub &&