QPointer: remove user-declared empty destructor

It prevents the compiler from synthesizing move assignment
and move constructor.

Change-Id: I864d143d5a6233e45f3f2fc343a147db89559f33
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/kernel/qpointer.h

@@ -64,7 +64,7 @@ public:
     inline QPointer() { }
     inline QPointer(T *p) : wp(p, true) { }
     // compiler-generated copy/move ctor/assignment operators are fine!
-    inline ~QPointer() { }
+    // compiler-generated dtor is fine!
 
     inline void swap(QPointer &other) { wp.swap(other.wp); }