LU-9859 libcfs: remove workitem.

LU-9859 libcfs: remove workitem.

There are no more users of the "workitem" code so it can be removed.
Lustre uses Linux workqueues instead.

Test-Parameters: trivial
Signed-off-by: Mr NeilBrown <neilb@suse.de>
Change-Id: I392ab73cdd38bc2927e0626d83a6f838dfd8113c
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/50462
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Aurelien Degremont <adegremont@nvidia.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>