Remove not used any more ImapIdleChannelAdapter.receivingTask property

The logic for the `ImapIdleChannelAdapter` was changed to not use `TaskScheduler`. 
When that was done the `ScheduledFuture<?> receivingTask` from the `ImapIdleChannelAdapter` was not removed.

Author: filiphr

Diff for: spring-integration-mail/src/main/java/org/springframework/integration/mail/ImapIdleChannelAdapter.java

@@ -19,7 +19,6 @@
 import java.io.Serial;
 import java.util.List;
 import java.util.concurrent.Executor;
-import java.util.concurrent.ScheduledFuture;
 import java.util.function.Consumer;
 
 import jakarta.mail.Folder;
@@ -79,8 +78,6 @@ public class ImapIdleChannelAdapter extends MessageProducerSupport implements Be
 
 	private long reconnectDelay = DEFAULT_RECONNECT_DELAY; // milliseconds
 
-	private volatile ScheduledFuture<?> receivingTask;
-
 	public ImapIdleChannelAdapter(ImapMailReceiver mailReceiver) {
 		Assert.notNull(mailReceiver, "'mailReceiver' must not be null");
 		this.mailReceiver = mailReceiver;
@@ -171,10 +168,6 @@ protected void doStart() {
 	@Override
 	// guarded by super#lifecycleLock
 	protected void doStop() {
-		if (this.receivingTask != null) {
-			this.receivingTask.cancel(true);
-			this.receivingTask = null;
-		}
 		this.mailReceiver.cancelPing();
 	}