Prevent use of deleted wxDocMDIChildFrame in wxView dtor

Revert 0d8737fd, it became unnecessary after a9e2e6e5 and prevents b7341fe0 from working properly
See #5066

include/wx/docview.h

@@ -722,9 +722,6 @@ public:
 
     virtual bool Destroy() wxOVERRIDE
     {
-        // FIXME: why exactly do we do this? to avoid activation events during
-        //        destructions maybe?
-        m_childView = NULL;
         return BaseClass::Destroy();
     }