window-actor: Don't queue a redraw when queueing a new pixmap

We guarantee ourselves that a valid pixmap will appear any time
that the window is painted. The window actor will be scheduled
for a repaint if it's added / removed from the scene graph, like
during construction, if the size changes, or if we receive damage,
which are the existing use cases where this function is called.

So, I can't see any reason that we queue a redraw in here.

With the split into surface actors, we don't have an easy place
we can use to queue a redraw, and since it's unnecessary, we can
just drop it on the floor.

https://bugzilla.gnome.org/show_bug.cgi?id=720631