scsi: virtio_scsi: unify scsi_host_template

Now that virtio_scsi uses blk-mq exclusively, we can remove the
scsi_host_template and associated plumbing for the legacy I/O path.

[mkp: commit desc]

Cc: Omar Sandoval <osandov@fb.com>,
Cc: "Martin K. Petersen" <martin.petersen@oracle.com>,
Cc: James Bottomley <james.bottomley@hansenpartnership.com>,
Cc: Christoph Hellwig <hch@lst.de>,
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Mike Snitzer <snitzer@redhat.com>
Cc: Laurence Oberman <loberman@redhat.com>
Cc: Hannes Reinecke <hare@suse.de>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Suggested-by: Christoph Hellwig <hch@lst.de&gt;,>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>