ipv6: remove unused in6_addr struct

struct in6_addr isn't used anymore in inet6_connection_sock.h, removing
the forward declaration.

Fixes: 1b33bc3e ("ipv6: remove obsolete inet6 functions")
Signed-off-by: Luis de Bethencourt <luisbg@osg.samsung.com>
Signed-off-by: David S. Miller <davem@davemloft.net>