Remove old compatibility code
The `mrb_voidp()` related macros have been removed in mruby-1.1.0.
#define E_SOCKET_ERROR mrb_class_get_id(mrb, MRB_SYM(SocketError))
-#if !defined(mrb_cptr)
-#define mrb_cptr_value(m,p) mrb_voidp_value((m),(p))
-#define mrb_cptr(o) mrb_voidp(o)
-#define mrb_cptr_p(o) mrb_voidp_p(o)
-#endif
-
#ifdef _WIN32
static const char *inet_ntop(int af, const void *src, char *dst, socklen_t cnt)
{
