9p: client: remove unused code and any reference to "cancelled" function

This patch reverts commit

80b45261a0b263536b043c5ccfc4ba4fc27c2acc

which was implementing a 'cancelled' functionality to notify that
a cancelled request will not be replied.

This implementation was not used anywhere and therefore removed.

Signed-off-by: Andi Shyti <andi@etezian.org>
Signed-off-by: David S. Miller <davem@davemloft.net>