Remove the now unnecessary empty wxMessageDialog dtor

No real changes, just some minor cleanup after 220dfe17ea (Replace
deprecated API usage for window modal dialogs, 2020-07-09).

include/wx/osx/msgdlg.h

@@ -21,10 +21,6 @@ public:
                     long style = wxOK|wxCENTRE,
                     const wxPoint& pos = wxDefaultPosition);
 
-#if wxOSX_USE_COCOA
-    ~wxMessageDialog();
-#endif
-
     virtual int ShowModal() wxOVERRIDE;
 
 #if wxOSX_USE_COCOA

src/osx/cocoa/msgdlg.mm

@@ -56,10 +56,6 @@ wxMessageDialog::wxMessageDialog(wxWindow *parent,
 {
 }
 
-wxMessageDialog::~wxMessageDialog()
-{
-}
-
 int wxMessageDialog::ShowModal()
 {
     WX_HOOK_MODAL_DIALOG();