net: Remove references to wan-router.txt in Kconfigs

This patch removes references in drivers/net/wan/Kconfig and
net/wanrouter/Kconfig to Documentation/networking/wan-router.txt
which was removed in commit 99971e70
("[WANPIPE]: Forgotten bits of Sangoma drivers removal.").

Signed-off-by: Johann Felix Soden <johfel@users.sourceforge.net>
Signed-off-by: David S. Miller <davem@davemloft.net>