LU-5814 lov: remove unused code

Remove:
  the tested but never set flag OBD_STATFS_PTLRPCD,
  the empty file lustre/lov/lovsub_io.c,
  the unused ld_emerg member of struct lov_device,
  the unused struct lov_device_emerg and supporting functions,
  the unused struct lov_lock_link and supporting functions, and
  the unused, get only, or set only members of struct
    lovsub_device, lovsub_lock, lov_sublock_env, lov_thread_info,
    lov_io_sub, lov_io, lov_request, and lov_request_set.

Reduce the scope of several functions from lov_request.c.

Signed-off-by: John L. Hammond <john.hammond@intel.com>
Change-Id: I3f4e7c94b401b5f90f7577c6c65311bd586a06ba
Reviewed-on: http://review.whamcloud.com/14878
Reviewed-by: Frank Zago <fzago@cray.com>
Reviewed-by: Ben Evans <bevans@cray.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>