Don't clear the pointer in QScopedPointer's destructor

It helps in valgrinding applications. It's the difference between:
==17609==  Address 0x38 is not stack'd, malloc'd or (recently) free'd

and
==19789==  Address 0x598f478 is 56 bytes inside a block of size 112 free'd
==19789==    at 0x4A0736C: operator delete(void*) (vg_replace_malloc.c:480)
==19789==    by 0x53A77CD: QObjectPrivate::~QObjectPrivate() (qobject.cpp:239)
==19789==    by 0x53B4EB5: QScopedPointerDeleter<QObjectData>::cleanup(QObjectData*) (qscopedpointer.h:63)
==19789==    by 0x53B3980: QScopedPointer<QObjectData, QScopedPointerDeleter<QObjectData> >::~QScopedPointer() (qscopedpointer.h:99)
==19789==    by 0x53A8EEC: QObject::~QObject() (qobject.cpp:750)

Change-Id: If42107c94401a96c05caa511442d6bd010fd4e29
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@digia.com>

src/corelib/tools/qscopedpointer.h

@@ -97,7 +97,6 @@ public:
     {
         T *oldD = this->d;
         Cleanup::cleanup(oldD);
-        this->d = 0;
     }
 
     inline T &operator*() const