ipv6: remove obsolete comment in ip6_append_data()

After 11878b40[net-timestamp: SOCK_RAW and PING timestamping], this comment
becomes obsolete since the codes check not only UDP socket, but also RAW sock;
and the codes are clear, not need the comments

Signed-off-by: Li RongQing <roy.qing.li@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>