Remove wxOVERRIDE not available in 3.0 branch.

Fixes compilation after backporting wxHTTP-in-worker-thread fixes from master.

See #17031.

samples/sockets/client.cpp

@@ -645,7 +645,7 @@ void MyFrame::OnTestURL(wxCommandEvent& WXUNUSED(event))
             Run();
         }
 
-        virtual void* Entry() wxOVERRIDE
+        virtual void* Entry()
         {
             DoDownload(m_url);