virtio-scsi: clean up virtio_scsi_handle_cmd_vq()

virtio_scsi_handle_cmd_vq() is only called from hw/scsi/virtio-scsi.c
now and its return value is no longer used. Remove the function
prototype from virtio-scsi.h and drop the return value.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 20220427143541.119567-6-stefanha@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>