vhost: remove unused lock check flag in vhost_dev_cleanup()

In commit ea5d4046 ("vhost: fix release path lockdep checks"),
Michael added a flag to check whether we should hold a lock in
vhost_dev_cleanup(), however, in commit 47283bef ("vhost: move
memory pointer to VQs"), RCU operations have been replaced by
mutex, we can remove the no-longer-used `locked' parameter now.

Signed-off-by: Caspar Zhang <jinli.zjl@alibaba-inc.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>