Documentation: decnet: remove reference to CONFIG_DECNET_ROUTE_FWMARK

CONFIG_DECNET_ROUTE_FWMARK was removed in commit 47dcf0cb

 ("[NET]: Rethink mark field in struct flowi")
Since nothing replace it (and nothindg need to replace it, simply remove
it from documentation.

Signed-off-by: Corentin Labbe <clabbe@baylibre.com>
Signed-off-by: David S. Miller <davem@davemloft.net>