ipv4: obsolete config in kernel source (IP_ROUTE_PERVASIVE)

CONFIG_IP_ROUTE_PERVASIVE is missing a corresponding config
IP_ROUTE_PERVASIVE somewhere in KConfig (and missing it for ages
already) so it looks like some aging artefact no longer needed.

Therefor this patch kills of the only remaining reference to that
config Item removing the already unrechable code snipet.

Signed-off-by: Christoph Egger <siccegge@stud.informatik.uni-erlangen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>