Taskmanager: Remove clean_shutdown (#10314)

There is no reason for this function, tokio already blocks automatically until all tasks are ended.
Another reason to remove this feature is `mpsc_background_tasks` unbounded channel. Recently this
channel was reporting too many unprocessed elements. We assume that this was a result of a lot of
very shot lived tasks that somehow flooded this channel.

substrate/test-utils/test-runner/src/node.rs

@@ -261,8 +261,6 @@ where
 			let signal = tokio::signal::ctrl_c();
 			futures::pin_mut!(signal);
 			futures::future::select(task, signal).await;
-			// we don't really care whichever comes first.
-			task_manager.clean_shutdown().await
 		}
 	}
 }