Removed unused inet6 address state

the inet6 state INET6_IFADDR_STATE_UP only appeared in its definition.

Cc: Christoph Paasch <christoph.paasch@uclouvain.be>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Sébastien Barré <sebastien.barre@uclouvain.be>
Signed-off-by: David S. Miller <davem@davemloft.net>