scsi: virtio_scsi: Remove per-target data because it is no longer used

Commit b5b6e8c8

 ("scsi: virtio_scsi: fix IO hang caused by automatic
irq vector affinity") removed all virtio_scsi hostdata users. Since the
SCSI host data is no longer used, also remove the host data itself.

Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Ming Lei <ming.lei@redhat.com>
Cc: Hannes Reinecke <hare@suse.de>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>