The vtkClipClosedSurface helper class vtkEdgeLocatorNode would
delete the whole node list in its destructor.  This is a bad
idea, because because if the implicit copy constructor is called
on a node, and the copy destructs, then the list will partially
destruct.  Add a FreeList() method that is called explicitly when
the list is to be freed.

Change-Id: I8945dc1682b6a70f66561b1f4155ed22be6fb0d6