Raised for each BoundedPlane that has been removed in the current frame.
A plane may be removed for two reasons:
1. The system determines the plane is no longer valid
2. The system determines two planes are really the same surface. In this case, one plane may be merged into the other.
The PlaneRemoved event is raised for one plane, and PlaneRemovedEventArgs.SubsumedByPlane will contain the
plane it was merged into.
See Also: BoundedPlane, PlaneRemovedEventArgs