LU-6742 osd: remove legacy code

LU-6742 osd: remove legacy code

osd_convert_root_to_new_seq() was introduced to convert
the filesystems created with pre-production Orion code.
we don't need this anymore.

Change-Id: Ib5657fc7a905bebacd4074878cc76365e1e3d8d9
Signed-off-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-on: http://review.whamcloud.com/15335
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Mike Pershin <mike.pershin@intel.com>
Reviewed-by: Nathaniel Clark <nathaniel.l.clark@intel.com>