Delete pointless discardShape method calls

Summary:
There's no point for calling discardShape from the destructor of a
Scene::Window subclass because window quads cache will be destroyed
automagically by QScopedPointer.

Reviewers: #kwin

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D24317

plugins/scenes/qpainter/scene_qpainter.cpp

@@ -223,7 +223,6 @@ SceneQPainter::Window::Window(SceneQPainter *scene, Toplevel *c)
 
 SceneQPainter::Window::~Window()
 {
-    discardShape();
 }
 
 static void paintSubSurface(QPainter *painter, const QPoint &pos, QPainterWindowPixmap *pixmap)

plugins/scenes/xrender/scene_xrender.cpp

@@ -327,7 +327,6 @@ SceneXrender::Window::Window(Toplevel* c, SceneXrender *scene)
 
 SceneXrender::Window::~Window()
 {
-    discardShape();
 }
 
 void SceneXrender::Window::cleanup()