block: remove dead struc request->completion_data field

It's no longer used. While in there, also update the comment as to why
it can coexist with the rb_node.
Signed-off-by: Jens Axboe <axboe@kernel.dk>

include/linux/blk-mq.h

@@ -158,13 +158,13 @@ struct request {
 
 	/*
 	 * The rb_node is only used inside the io scheduler, requests
-	 * are pruned when moved to the dispatch queue. So let the
-	 * completion_data share space with the rb_node.
+	 * are pruned when moved to the dispatch queue. special_vec must
+	 * only be used if RQF_SPECIAL_PAYLOAD is set, and those cannot be
+	 * insert into an IO scheduler.
 	 */
 	union {
 		struct rb_node rb_node;	/* sort/lookup */
 		struct bio_vec special_vec;
-		void *completion_data;
 	};
 
 	/*