staging/lustre/ptlrpc: Remove log message about export timer update

Function ptlrpc_update_export_timer generates lots of D_HA level log
messages whenever the export timer gets updated. Those log messages
are found little use for issue investigations, and it will take space
in the Lustre log buffer. We are removing it now.

Xyratex-bug-id: MRP-733
Signed-off-by: Cheng Shao <cheng_shao@xyratex.com>
Reviewed-on: http://review.whamcloud.com/9147
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-4590


Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Signed-off-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>