QSimplex: make non-polymorphic

This class is never inherited from, so save the vtable, and
everything that goes with it (typeinfo, ...).

Effects on Linux AMD64 GCC 4.9-trunk release stripped:
  text:   -248B
  data:    -64B
  relocs:   -5

Change-Id: I12eec1115529eb3fc873fb9a9aacc150d9937fb9
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>

src/widgets/graphicsview/qsimplex_p.h

@@ -152,7 +152,7 @@ class QSimplex
     Q_DISABLE_COPY(QSimplex)
 public:
     QSimplex();
-    virtual ~QSimplex();
+    ~QSimplex();
 
     qreal solveMin();
     qreal solveMax();