ipv6: remove unused function ipv6_skb_idev()

Commit bdb7cc64 ("ipv6: Count interface receive statistics on the
ingress netdev") removed all callees for ipv6_skb_idev(). Hence, since
then, ipv6_skb_idev() is unused and make CC=clang W=1 warns:

  net/ipv6/exthdrs.c:909:33:
    warning: unused function 'ipv6_skb_idev' [-Wunused-function]

So, remove this unused function and a -Wunused-function warning.
Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>
Link: https://lore.kernel.org/r/20201113135012.32499-1-lukas.bulwahn@gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/ipv6/exthdrs.c

@@ -906,11 +906,6 @@ void ipv6_exthdrs_exit(void)
 /*
  * Note: we cannot rely on skb_dst(skb) before we assign it in ip6_route_input().
  */
-static inline struct inet6_dev *ipv6_skb_idev(struct sk_buff *skb)
-{
-	return skb_dst(skb) ? ip6_dst_idev(skb_dst(skb)) : __in6_dev_get(skb->dev);
-}
-
 static inline struct net *ipv6_skb_net(struct sk_buff *skb)
 {
 	return skb_dst(skb) ? dev_net(skb_dst(skb)->dev) : dev_net(skb->dev);