Break reference cycles in vtk pipeline

• With this MR, the VTK pipeline no longer requires garbage collection.
• Previously an executive stored itself and its consumers in an information object that was shared among the producer and consumer and caused reference loops.
• Whereas now, a consumer keeps its producers alive by maintaining an internal list of its input algorithms and their output port indices. See vtkAlgorithmInternals::InputAlgorithms.
• vtkExecutive::PRODUCER() and vtkExecutive::CONSUMERS() are no longer required and those keys are marked deprecated.
• Aditionally, the vtkInformationExecutivePortKey and vtkInformationExecutivePortVectorKey classes are not used anymore.
• The algorithm <-> executive reference loop is manually severed in their respective ::UnRegisterInternal methods.