Qt: do not delete speedControl yourself

It is a QFrame, so it is already parented and should work fine.
Fixes crash-on-quit in Qt5

modules/gui/qt4/components/interface_widgets.cpp

@@ -487,7 +487,6 @@ SpeedLabel::SpeedLabel( intf_thread_t *_p_intf, QWidget *parent )
 
 SpeedLabel::~SpeedLabel()
 {
-    delete speedControl;
     delete speedControlMenu;
 }