ipv6: remove old conditions on flow label sharing

The code of flow label in Linux Kernel follows
the rules of RFC 1809 (an informational one) for
conditions on flow label sharing. There rules are
not in the last proposed standard for flow label
(RFC 6437), or in the previous one (RFC 3697).

Since this code does not follow any current or
old standard, we can remove it.

With this removal, the ipv6_opt_cmp function is
now a dead code and it can be removed too.

Changelog to v1:
 * add justification for the change
 * remove the condition on IPv6 options

[ Remove ipv6_hdr_cmp and it is now unused as well. -DaveM ]

Signed-off-by: Florent Fourcot <florent.fourcot@enst-bretagne.fr>
Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>