vhost: remove vhost_work_queue

vhost_work_queue is no longer used. Each driver is using the poll or vq
based queueing, so remove vhost_work_queue.

Signed-off-by: Mike Christie <michael.christie@oracle.com>
Message-Id: <20230626232307.97930-13-michael.christie@oracle.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>