针对已在当前帧中删除的每个 BoundedPlane 引发。
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 The system determines the plane is no longer valid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