examples/vhost_scsi: drop unimplemented event index feature

examples/vhost_scsi: drop unimplemented event index feature

The vhost_scsi example application negotiates the
VIRTIO_RING_F_EVENT_IDX feature bit but does not honor it when accessing
vrings.

In particular, commit e37ff954405addb8ea422426a2d162d00dcad196 ("vhost:
support virtqueue interrupt/notification suppression") broke vring call
because vq->last_used_idx is never updated by vhost_scsi.  The
vq->last_used_idx field is not even available via the librte_vhost
public API, so VIRTIO_RING_F_EVENT_IDX is currently only usable by the
built-in virtio_net.c driver in librte_vhost.

This patch drops VIRTIO_RING_F_EVENT_IDX from vhost_scsi so that vring
call works again.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Changpeng Liu <changpeng.liu@intel.com>
Acked-by: Yuanhan Liu <yliu@fridaylinux.org>