Remove unnecessary wxSearchCtrl::OnSetFocus()

This was useless as wxCompositeWindow, from which the generic
wxSearchCtrl derives, already sets focus to its first child and, since
the last commit, even harmful as it now resulted in calls to SetFocus()
from inside wxEVT_SET_FOCUS handler which is forbidden at least under
MSW.

See #15569.

include/wx/generic/srchctlg.h

@@ -223,7 +223,6 @@ protected:
 
     void OnCancelButton( wxCommandEvent& event );
 
-    void OnSetFocus( wxFocusEvent& event );
     void OnSize( wxSizeEvent& event );
 
     bool HasMenu() const