kiosk: remove invalid unref

This unref doesn't seem to be related to any reference, although it
was probably introduced in the first place to clear the floating ref,
wrongly. See following commit for a working solution.

src/virt-viewer-app.c

@@ -844,7 +844,6 @@ virt_viewer_app_display_added(VirtViewerSession *session G_GNUC_UNUSED,
         if (priv->kiosk) {
             /* don't show extra monitors that don't fit on client */
             g_debug("kiosk mode: skip extra monitors that don't fit on client");
-            g_object_unref(display);
             return;
         }