window-actor/wayland: Remove get_paint_volume() vfunc

It doesn't take all children - subsurfaces in this case - into account, thus creating glitches if subsurfaces extend outside of the toplevel surface.

Further more it doesn't seem to serve any special purpose - it was added in f7315c9a, a pretty big commit, and no discussion was started about the code in question. So it was likely just overlooked in the review process.

Closes #873 (closed) Closes #1316 (closed)