wxX11: don't call CleanUp() from wxApp::Exit()

This is not done in any other ports and shouldn't be necessary in this
one neither, CleanUp() will be called from wxEntryCleanup() later
anyhow.

include/wx/x11/app.h

@@ -41,8 +41,6 @@ public:
     // override base class (pure) virtuals
     // -----------------------------------
 
-    virtual void Exit();
-
     virtual void WakeUpIdle();
 
     virtual bool OnInitGui();