[oxygen] Drop redefining the default decoration buttons

There is no difference between Oxygen and default KDecoration, so
it's not needed. It's also conceptionally wrong: decorations should
not be able to change the global default.

clients/oxygen/oxygenclient.cpp

@@ -1327,14 +1327,6 @@ namespace Oxygen
 
     }
 
-    //_________________________________________________________
-    QString Client::defaultButtonsLeft() const
-    { return KCommonDecoration::defaultButtonsLeft(); }
-
-    //_________________________________________________________
-    QString Client::defaultButtonsRight() const
-    { return QStringLiteral("HIAX"); }
-
     //________________________________________________________________
     void Client::updateWindowShape()
     {

clients/oxygen/oxygenclient.h

@@ -192,12 +192,6 @@ namespace Oxygen
 
         //@}
 
-        //! default buttons located on the left
-        virtual QString defaultButtonsLeft() const;
-
-        //! default buttons located on the right
-        virtual QString defaultButtonsRight() const;
-
         //! title alignment
         inline Qt::Alignment titleAlignment( void ) const;