net: Remove unused skb_shared_info member

ip6_frag_id was only used by UFO, which has been removed.
ipv6_proxy_select_ident() only existed to set ip6_frag_id and has no
in-tree callers.

Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>