MRP-733 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.

Signed-off-by: Cheng Shao <cheng_shao@xyratex.com>
Change-Id: I1c917abf04f777b3087e6e5bf487c18d41dcfbfc
Reviewed-on: http://morpheus.xyus.xyratex.com:8443/gerrit/561
Tested-by: Jenkins
Reviewed-by: Nathan Rutman <Nathan_Rutman@xyratex.com>
Reviewed-by: Alexander Boyko <Alexander_Boyko@xyratex.com>
Tested-by: Alexander Lezhoev <Alexander_Lezhoev@xyratex.com>
Reviewed-by: Vitaly Fertman <Vitaly_Fertman@xyratex.com>