LU-14838 osc: Remove lockless truncate

LU-14838 osc: Remove lockless truncate

Lockless truncate does not work and cannot be made to work.

Fundamentally, it has no means of ensuring consistency
across clients because it can't force them all to drop
cached data without locking.

It's been off for years - let's just get rid of it.

Lustre-change: https://review.whamcloud.com/44204
Lustre-commit: 6335dba83995765c1ffcd7993eb8958c162913e1

Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: Ia2979fb6b31a61da6d4833e9f463fcd5b6dbd718
Reviewed-on: https://review.whamcloud.com/44436
Reviewed-by: Bobi Jam <bobijam@hotmail.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>