Remove Scene::Window::{discard,update}Pixmap

They are used only by X11Client, so make X11Client call relevants
methods on the surface item directly instead. In hindsight, it will be a
really good idea to make SurfaceItemX11 and SurfaceItemXwayland(?)
automatically manage the window pixmap. However, it can be done once
an item freezing api is added and we fix the cross-fade animation.