Remove unused vdev_t fields

The following fields from the vdev_t struct are not used anywhere.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: Serapheim Dimitropoulos <serapheim@delphix.com>
Closes #8285

include/sys/vdev_impl.h

@@ -254,7 +254,6 @@ struct vdev {
 	uint64_t	vdev_islog;	/* is an intent log device	*/
 	uint64_t	vdev_removing;	/* device is being removed?	*/
 	boolean_t	vdev_ishole;	/* is a hole in the namespace	*/
-	kmutex_t	vdev_queue_lock; /* protects vdev_queue_depth	*/
 	uint64_t	vdev_top_zap;
 	vdev_alloc_bias_t vdev_alloc_bias; /* metaslab allocation bias	*/
 
@@ -314,16 +313,6 @@ struct vdev {
 	range_tree_t	*vdev_obsolete_segments;
 	space_map_t	*vdev_obsolete_sm;
 
-	/*
-	 * The queue depth parameters determine how many async writes are
-	 * still pending (i.e. allocated but not yet issued to disk) per
-	 * top-level (vdev_async_write_queue_depth) and the maximum allowed
-	 * (vdev_max_async_write_queue_depth). These values only apply to
-	 * top-level vdevs.
-	 */
-	uint64_t	vdev_async_write_queue_depth;
-	uint64_t	vdev_max_async_write_queue_depth;
-
 	/*
 	 * Protects the vdev_scan_io_queue field itself as well as the
 	 * structure's contents (when present).