Revive route cleanup for /sbin/route code

Revive route cleanup for /sbin/route code

Cleanup our routes upon disconnect also when using /sbin/route command - even
if OS will do it for us when TUNDEV gets removed. That's the same what the
ip-route code already does.

Additionally, we obviously didn't need route deletion in set_network_route for
/sbin/route mode - this function didn't do anything until now. So remove this
call -- it's a bad idea to call del_network_route inside set_network_route
anyways because both modify the same global variables.

Signed-off-by: Gernot Hillier <gernot.hillier@siemens.com>
Tested-by: Thomas Zander <thomas.zander@siemens.com>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>