net: Remove DECnet leftovers from flow.h.

DECnet was removed by commit 1202cdd6 ("Remove DECnet support from
kernel"). Let's also revome its flow structure.

Compile-tested only (allmodconfig).

Signed-off-by: Guillaume Nault <gnault@redhat.com>
Acked-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: David S. Miller <davem@davemloft.net>