vti6: Remove caching of flow informations.

Unlike ip6_tunnel, vti6 does not use the the tunnel
endpoint addresses to do route and xfrm lookups.
So no need to cache the flow informations. It also
does not make sense to calculate the mtu based on
such flow informations, so remove this too.

Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>