Use qDeleteAll in destructor of Scene class

Summary:
qDeleteAll can be used to delete values in QMap or QHash. Please notice
that qDeleteAll calls `delete` only for values, not keys.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D13822

scene.cpp

@@ -100,9 +100,7 @@ Scene::Scene(QObject *parent)
 
 Scene::~Scene()
 {
-    foreach (Window *w, m_windows) {
-        delete w;
-    }
+    qDeleteAll(m_windows);
 }
 
 // returns mask and possibly modified region