net: update Documentation/networking/00-INDEX

Clean up entries in 00-INDEX: drop files that have been removed.

Reported-by: Rob Landley <rlandley@parallels.com>
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Acked-by: Rob Landley <rlandley@parallels.com>
Signed-off-by: David S. Miller <davem@davemloft.net>