LU-2484 mgs: remove unused md_stats

LU-2484 mgs: remove unused md_stats

Do not allocate md_stats or create /proc/.../MGS/md_stats for mgs
devices since they do not implement the md_ops interface.

Signed-off-by: John L. Hammond <john.hammond@intel.com>
Change-Id: I8b417bc2433484a0e008ff9bf7fef69ba1e62416
Reviewed-on: http://review.whamcloud.com/4813
Tested-by: Hudson
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Keith Mannthey <keith.mannthey@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>