ip6_udp_tunnel: remove unused IPCB related codes

Some IPCB fields are currently set in udp_tunnel6_xmit_skb(), which are
never used before it reaches ip6tunnel_xmit(), and past that point the
control buffer is no longer interpreted as IPCB.

This clears these unused IPCB related codes. Currently there is no skb
scrubbing in ip6_udp_tunnel, otherwise IPCB(skb)->opt might need to be
cleared for IPv4 packets, as shown in 5146d1f1
("tunnel: Clear IPCB(skb)->opt before dst_link_failure called").

Signed-off-by: Eli Cooper <elicooper@gmx.com>
Signed-off-by: David S. Miller <davem@davemloft.net>