unix: don't close kqueue fd in kqueue_fork()

The file descriptor is not inherited by the child process. Trying to close it is
not just useless, it's actively harmful - if the close() call succeeds, it will
have closed an altogether different file descriptor.

src/unix/ev/ev_kqueue.c

@@ -200,8 +200,6 @@ kqueue_destroy (EV_P)
 void inline_size
 kqueue_fork (EV_P)
 {
-  close (backend_fd);
-
   while ((backend_fd = kqueue ()) < 0)
     ev_syserr ("(libev) kqueue");