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 99971e70fdc1862e120f3319fc0a4dba8c728acf
("[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>
diff --git a/drivers/net/wan/Kconfig b/drivers/net/wan/Kconfig
index d5140ae..846be60 100644
--- a/drivers/net/wan/Kconfig
+++ b/drivers/net/wan/Kconfig
@@ -390,8 +390,7 @@
 
 	  Select driver your card and remember to say Y to "Wan Router."
 	  You will need the wan-tools package which is available from
-	  <ftp://ftp.sangoma.com/>. For more information read:
-	  <file:Documentation/networking/wan-router.txt>.
+	  <ftp://ftp.sangoma.com/>.
 
 	  Note that the answer to this question won't directly affect the
 	  kernel except for how subordinate drivers may be built: