Gui: modernize C++: use equals default

src/Gui/Selection.cpp

@@ -1625,9 +1625,7 @@ SelectionSingleton::SelectionSingleton()
  * A destructor.
  * A more elaborate description of the destructor.
  */
-SelectionSingleton::~SelectionSingleton()
-{
-}
+SelectionSingleton::~SelectionSingleton() = default;
 
 SelectionSingleton* SelectionSingleton::_pcSingleton = nullptr;