Remove an unused `#if 0` code

vm_exec.c

@@ -39,18 +39,6 @@ static void vm_analysis_insn(int insn);
 static VALUE
 vm_exec_core(rb_execution_context_t *ec, VALUE initial)
 {
-
-#if OPT_STACK_CACHING
-#if 0
-#elif __GNUC__ && __x86_64__
-    DECL_SC_REG(VALUE, a, "12");
-    DECL_SC_REG(VALUE, b, "13");
-#else
-    register VALUE reg_a;
-    register VALUE reg_b;
-#endif
-#endif
-
 #if defined(__GNUC__) && defined(__i386__)
     DECL_SC_REG(const VALUE *, pc, "di");
     DECL_SC_REG(rb_control_frame_t *, cfp, "si");