cls/log/cls_log.cc: reduce logging noise

cls/log/cls_log.cc: reduce logging noise

 - The other reference in the source as already at 20.
      ./src/cls/timeindex/cls_timeindex.cc:85:
	CLS_LOG(20, "storing entry at %s", index.c_str());

   And we need not always know where in the log items are stored.
   So it looks like a leftover debug feature.

Fixes: http://tracker.ceph.com/issues/19835
Signed-off-by: Willem Jan Withagen <wjw@digiware.nl>
(cherry picked from commit d76010900bf9012f2e66335787710531772766b7)