block: genhd: remove async_events field

The async_events field, intended to be used for drivers that support
asynchronous notifications about disk events (aka media change events),
isn't currently used by any driver, and apparently that has been that
way for a long time (if not forever). Remove it.

Reviewed-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Martin Wilck <mwilck@suse.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

include/linux/genhd.h

@@ -185,7 +185,6 @@ struct gendisk {
 	char *(*devnode)(struct gendisk *gd, umode_t *mode);
 
 	unsigned int events;		/* supported events */
-	unsigned int async_events;	/* async events, subset of all */
 
 	/* Array of pointers to partitions indexed by partno.
 	 * Protected with matching bdev lock but stat and other