netfilter: Remove references to obsolete CONFIG_IP_ROUTE_FWMARK

This option was removed in commit 47dcf0cb ("[NET]: Rethink mark field
in struct flowi").

Signed-off-by: Moritz Sichert <moritz+linux@sichert.me>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>