scripts/desktopchangeosd: Minor cleanup

Workspace.desktops is a property so the binding will be updated
automatically when the desktops change. There's also no a signal with
name "numberDesktopsChanged"

src/scripts/desktopchangeosd/contents/ui/osd.qml

@@ -276,9 +276,6 @@ PlasmaCore.Dialog {
             function onCurrentDesktopChanged() {
                 dialogItem.show()
             }
-            function onNumberDesktopsChanged() {
-                repeater.model = Workspace.desktops;
-            }
         }
         Connections {
             target: Options