LU-6142 lustre: remove ldt_obd_type field of lu_device_type

LU-6142 lustre: remove ldt_obd_type field of lu_device_type

This field is never set, so it is always NULL.
So remove it,
 and the one place it is used,
 and a variable that now will now never be set.

Test-Parameters: trivial
Signed-off-by: Mr NeilBrown <neilb@suse.com>
Change-Id: I5530dd92b653b4259ddcbf8dc21af03a79ed255a
Reviewed-on: https://review.whamcloud.com/35876
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Shaun Tancheff <stancheff@cray.com>
Reviewed-by: Arshad Hussain <arshad.super@gmail.com>
Tested-by: Maloo <maloo@whamcloud.com>