ipv6: remove ipv6_statistics

commit 9261e537 (ipv6: making ip and icmp statistics per/namespace)
forgot to remove ipv6_statistics variable.

commit bc417d99

 (ipv6: remove stale MIB definitions) took care of
icmpv6_statistics & icmpv6msg_statistics

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
CC: Denis V. Lunev <den@openvz.org>
CC: Alexey Dobriyan <adobriyan@gmail.com>
CC: Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>