Gui: modernize C++: use equals default

src/Gui/DlgSettingsColorGradientImp.cpp

@@ -77,10 +77,7 @@ DlgSettingsColorGradientImp::DlgSettingsColorGradientImp(const App::ColorGradien
 /**
  *  Destroys the object and frees any allocated resources
  */
-DlgSettingsColorGradientImp::~DlgSettingsColorGradientImp()
-{
-    // no need to delete child widgets, Qt does it all for us
-}
+DlgSettingsColorGradientImp::~DlgSettingsColorGradientImp() = default;
 
 void DlgSettingsColorGradientImp::setupConnections()
 {