af_unix: remove unix_iter_state

As pointed out by Michael Tokarev , struct unix_iter_state is no longer
needed.

Suggested-by: Michael Tokarev <mjt@tls.msk.ru>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Steven Whitehouse <swhiteho@redhat.com>
Cc: Pavel Emelyanov <xemul@parallels.com>
Signed-off-by: David S. Miller <davem@davemloft.net>