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

Documentation/networking/decnet.txt

@@ -22,8 +22,6 @@ you'll need the following options as well...
     CONFIG_DECNET_ROUTER (to be able to add/delete routes)
     CONFIG_NETFILTER (will be required for the DECnet routing daemon)
 
-    CONFIG_DECNET_ROUTE_FWMARK is optional
-
 Don't turn on SIOCGIFCONF support for DECnet unless you are really sure
 that you need it, in general you won't and it can cause ifconfig to
 malfunction.