veth: Remove useless veth field

The veth private structure contains a netdev pointer refering to its peer.
This field is never used and it is pointless because if we can access,
the veth_priv, that means we already have the netdev which is stored
in veth_priv->dev.

Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>