md/dm - remove remains of plug_fn callback.

Now that unplugging is done differently, the unplug_fn callback is
never called, so it can be completely discarded.

Signed-off-by: NeilBrown <[e-mail]>

drivers/md/dm-raid.c

···
 	return md_raid5_congested(&rs->md, bits);
 }
 
-static void raid_unplug(struct dm_target_callbacks *cb)
-{
-	struct raid_set *rs = container_of(cb, struct raid_set, callbacks);
-
-	md_raid5_kick_device(rs->md.private);
-}
-
 /*
  * Construct a RAID4/5/6 mapping:
  * Args:
···
 	}
 
 	rs->callbacks.congested_fn = raid_is_congested;
-	rs->callbacks.unplug_fn = raid_unplug;
 	dm_table_add_target_callbacks(ti->table, &rs->callbacks);
 
 	return 0;

include/linux/device-mapper.h

···
 struct dm_target_callbacks {
 	struct list_head list;
 	int (*congested_fn) (struct dm_target_callbacks *, int);
-	void (*unplug_fn)(struct dm_target_callbacks *);
 };
 
 int dm_register_target(struct target_type *t);