ipv6: remove unused function ipv6_skb_idev()

ipv6: remove unused function ipv6_skb_idev()

Commit bdb7cc643fc9 ("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.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>