staging/lustre/obd: Remove nid_stats tracking

This nid_stats tracking only makes sense on the server side,
on the client there are no other clients to keep track of anyway.

Signed-off-by: Dmitry Eremin <dmiter4ever@gmail.com>
Signed-off-by: Oleg Drokin <green@linuxhacker.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>