LU-10175 ldlm: remove obsoleted lock convert code

LU-10175 ldlm: remove obsoleted lock convert code

Patch removes lock mode convert mechanics from Lustre,
it is obsoleted and not functional at the moment. Also
there are no plans to restore it and use again.

Signed-off-by: Mikhail Pershin <mike.pershin@intel.com>
Change-Id: I477caf24927768dfcdc15888e59a7d5e62d5b577
Reviewed-on: https://review.whamcloud.com/30491
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Bobi Jam <bobijam@hotmail.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>