snmp6: remove IPSTATS_MIB_CSUMERRORS

This stat is not relevant in IPv6, there is no checksum in IPv6 header.
Just leave a comment to explain the hole.

Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>