DOC: Override destructor and prefer = default for trivial bodies

Mark class destructor with `override` and replace its default body with
= default; to match current practice in ITK.

Cross-referencing related ITK commits, e.g.:
InsightSoftwareConsortium/ITK@7ef2496
InsightSoftwareConsortium/ITK@9b74640

Author: jhlegarreta

SoftwareGuide/Latex/Appendices/CodingStyleGuide.tex

@@ -1865,14 +1865,6 @@ \subsection{Class Layout}
   m_Buffer = PixelContainer::New();
 }
 
-
-template< typename TPixel, unsigned int VImageDimension >
-Image< TPixel, VImageDimension >
-~::Image()
-{
-}
-
-
 ...
 
 
@@ -2712,7 +2704,7 @@ \subsection{Empty Lines}
 protected:
   BoxImageFilter();
 
-  ~BoxImageFilter() {}
+  ~BoxImageFilter() override = default;
 
   void GenerateInputRequestedRegion() override;