scsi: make asynchronous aborts mandatory

There hasn't been any reports for HBAs where asynchronous abort
would not work, so we should make it mandatory and remove
the fallback.

Signed-off-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Reviewed-by: Bart Van Assche <bart.vanassche@sandisk.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

include/scsi/scsi_host.h

@@ -451,11 +451,6 @@ struct scsi_host_template {
 	/* True if the controller does not support WRITE SAME */
 	unsigned no_write_same:1;
 
-	/*
-	 * True if asynchronous aborts are not supported
-	 */
-	unsigned no_async_abort:1;
-
 	/*
 	 * Countdown for host blocking with no commands outstanding.
 	 */