stackswitcher: Don't do unneeded update

The update of the needs-attention state is done via its own property,
so it doesn't need to be done via visibility changes.

This patch is largely the result of inspecting the code due to a warning
and not a result of testing. So if issues pop up that bisect back to
this patch, that's why.

gtk/gtkstackswitcher.c

@@ -310,10 +310,6 @@ on_child_changed (GtkWidget        *widget,
       gtk_toggle_button_set_active (GTK_TOGGLE_BUTTON (button), TRUE);
       priv->in_child_changed = FALSE;
     }
-
-  g_hash_table_foreach (priv->buttons,
-                        (GHFunc)update_needs_attention,
-                        priv->stack);
 }
 
 static void