Remove mention of ChildrenUpdated from the ListView docs

As pointed out in Textualize#1649, a `ChildrenUpdated` message is documented but it
doesn't exist in the code. It looks like it got added during development,
then removed after it was realised it wasn't needed, but presumably it got
left in the docs:

  Textualize#1143 (comment)

docs/widgets/list_view.md

@@ -64,21 +64,6 @@ or by clicking on it.
 | `item`    | `ListItem` | The item that was selected. |
 
 
-### ChildrenUpdated
-
-The `ListView.ChildrenUpdated` message is emitted when the elements in the `ListView`
-are changed (e.g. a child is added, or the list is cleared).
-
-- [x] Bubbles
-
-#### Attributes
-
-| attribute  | type             | purpose                   |
-| ---------- | ---------------- | ------------------------- |
-| `children` | `list[ListItem]` | The new ListView children |
-
-
-
 ## See Also
 
 * [ListView](../api/list_view.md) code reference