[S-EXP] Remove code for Common Lisp S-Exp

We dropped support for it in SWIG 4.0.0 and nobody has stepped forward
to revive it in over 2 years.

See #2009.

Examples/ocaml/shapes/example.i

@@ -1,10 +1,8 @@
 /* File : example.i */
 %module(directors="1") example
-#ifndef SWIGSEXP
 %{
 	#include "example.h"
 %}
-#endif
 
 %feature("director");
 %include "example.h"