COMP: FVLengthGUI: Fix warning: variable ‘Renderer’ set but not used [-Wunused-but-set-variable]
COMP: FVLengthGUI: Fix warning: variable ‘Renderer’ set but not used [-Wunused-but-set-variable]

This commit fixes the following warning:

```
/tmp/FiberViewerLight/FVLengthGUI.cxx: In member function ‘void FVLengthGUI::LengthComputation()’:
/tmp/FiberViewerLight/FVLengthGUI.cxx:123:15: warning: variable ‘Renderer’ set but not used [-Wunused-but-set-variable]
  vtkRenderer* Renderer=vtkRenderer::New();
               ^
```

NIRALUser/FiberViewerLight,NIRALUser/FiberViewerLight,NIRALUser/FiberViewerLight