staging/lustre/ptlrpc: Remove ptlrpc_update_export_timer()

This is only used on the server to keep track of alive clients
and feeds into ping evictor (that was removed from the client code).
Also remove struct obd's obd_exports_timed and
struct obd_export's exp_obd_chain_timed used to keep track of that

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