vhost: Remove the unused variable.

The patch (7235acdb) changed the way of the work
flushing in which the queued seq, done seq, and the
flushing are not used anymore. Then remove them now.

Fixes: 7235acdb ("vhost: simplify work flushing")
Cc: Jason Wang <jasowang@redhat.com>
Signed-off-by: Tonghao Zhang <xiangxia.m.yue@gmail.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>