These classes where using the macros vtkMRMLWriteXMLBeginMacro and
vtkMRMLWriteXMLEndMacro but without any content printed or written
between those calls.

This commit fixes warnings like the following:

```cpp
[...]
Modules/Loadable/VolumeRendering/MRML/vtkMRMLGPURayCastVolumeRenderingDisplayNode.cxx: In member function ‘virtual void vtkMRMLGPURayCastVolumeRenderingDisplayNode::ReadXMLAttributes(const char**)’:
Libs/MRML/Core/vtkMRMLNodePropertyMacros.h:148:15: warning: variable ‘xmlReadAttName’ set but not used [-Wunused-but-set-variable]
   const char* xmlReadAttName; \
               ^~~~~~~~~~~~~~
Modules/Loadable/VolumeRendering/MRML/vtkMRMLGPURayCastVolumeRenderingDisplayNode.cxx:48:3: note: in expansion of macro ‘vtkMRMLReadXMLBeginMacro’
   vtkMRMLReadXMLBeginMacro(atts);
   ^~~~~~~~~~~~~~~~~~~~~~~~
[...]
Modules/Loadable/VolumeRendering/MRML/vtkMRMLGPURayCastVolumeRenderingDisplayNode.cxx: In member function ‘virtual void vtkMRMLGPURayCastVolumeRenderingDisplayNode::WriteXML(std::ostream&, int)’:
Libs/MRML/Core/vtkMRMLNodePropertyMacros.h:30:12: warning: unused variable ‘xmlWriteOutputStream’ [-Wunused-variable]
   ostream& xmlWriteOutputStream = of;
            ^~~~~~~~~~~~~~~~~~~~
Modules/Loadable/VolumeRendering/MRML/vtkMRMLGPURayCastVolumeRenderingDisplayNode.cxx:57:3: note: in expansion of macro ‘vtkMRMLWriteXMLBeginMacro’
   vtkMRMLWriteXMLBeginMacro(of);
   ^~~~~~~~~~~~~~~~~~~~~~~~~
```

```cpp
[...]
Modules/Loadable/VolumeRendering/MRML/vtkMRMLGPURayCastVolumeRenderingDisplayNode.cxx: In member function ‘virtual void vtkMRMLGPURayCastVolumeRenderingDisplayNode::PrintSelf(std::ostream&, vtkIndent)’:
Libs/MRML/Core/vtkMRMLNodePropertyMacros.h:417:12: warning: unused variable ‘printOutputStream’ [-Wunused-variable]
   ostream& printOutputStream = os; \
            ^~~~~~~~~~~~~~~~~
Modules/Loadable/VolumeRendering/MRML/vtkMRMLGPURayCastVolumeRenderingDisplayNode.cxx:78:3: note: in expansion of macro ‘vtkMRMLPrintBeginMacro’
   vtkMRMLPrintBeginMacro(os, indent);
   ^~~~~~~~~~~~~~~~~~~~~~
Libs/MRML/Core/vtkMRMLNodePropertyMacros.h:418:13: warning: variable ‘printOutputIndent’ set but not used [-Wunused-but-set-variable]
   vtkIndent printOutputIndent = indent;
             ^~~~~~~~~~~~~~~~~
Modules/Loadable/VolumeRendering/MRML/vtkMRMLGPURayCastVolumeRenderingDisplayNode.cxx:78:3: note: in expansion of macro ‘vtkMRMLPrintBeginMacro’
   vtkMRMLPrintBeginMacro(os, indent);
```

From: Pablo Hernandez-Cerdan <pablo.hernandez@kitware.com>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@27999 3bd1e089-480b-0410-8dfb-8563597acbee