vnc: Remove scanout plane optimization

This is a revert of 72e2da24

The VNC backend will place a single fullscreen surface on a virtual
scanout plane, and send the entire contents of this plane every repaint.

This saves a renderer pass, but moving the mouse over the fullscreen
client results in full screen damage for every mouse motion, similarly
client surfact damage is ignored and every repaint pushes the entire
window content down into Neat VNC.

Due to the way this is implemented, by pushing the scanout plane content
from assign_planes(), the primary plane could post damage and corrupt
the display.

Ideally we could fix this optimization to respect plane damage and do the
scanout plane push from the repaint callback, but since a release is
coming soon let's just strip it out for now.

Signed-off-by: Derek Foreman <derek.foreman@collabora.com>