Revert "NET: Fix locking issues in PPP, 6pack, mkiss and strip line disciplines."

This reverts commit adeab1af

.

As Alan Cox explained, the TTY layer changes that went recently
to get rid of the tty->low_latency stuff fixes this already,
and even for -stable it's the ->low_latency changes that should
go in to fix this, rather than this patch.

Signed-off-by: David S. Miller <davem@davemloft.net>