src/usocket: Do not setblocking on destory;

This results in unexpected behaviour if the socket has been `dup()`d, as O_NONBLOCK is shared.
Close is always 'blocking' anyway

See wahern/cqueues#13 for an example use case

src/usocket.c

@@ -92,7 +92,6 @@ int socket_close(void) {
 \*-------------------------------------------------------------------------*/
 void socket_destroy(p_socket ps) {
     if (*ps != SOCKET_INVALID) {
-        socket_setblocking(ps);
         close(*ps);
         *ps = SOCKET_INVALID;
     }