Remove PIPEXCSESSION ioctl(2) call only from npppd(8).

Remove PIPEXCSESSION ioctl(2) call only from npppd(8).

Long time ago pipex(4) sessions can't be deleted until pipex(4) queues
become empty. Such dead sessions stay linked to the stack and the
session `ip_forward' flag was used to prevent packets processing. But
now pipex(4) session could be closed just after close request. This
logic became unnecessary.

PIPEXCSESSION ioctl(2) will be removed from kernel with upcoming diff.

ok yasuoka@