LU-2675 md: remove unused code from md_object.h

LU-2675 md: remove unused code from md_object.h

Remove several unused functions, structures, and members from
md_object.h.

Signed-off-by: John L. Hammond <john.hammond@intel.com>
Change-Id: I33de0ba987bfde95172e9bfb77929b6b4dcd0aa8
Reviewed-on: http://review.whamcloud.com/12113
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Fan Yong <fan.yong@intel.com>
Reviewed-by: Mike Pershin <mike.pershin@intel.com>
Reviewed-by: Dmitry Eremin <dmitry.eremin@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>