[FLINK-191909][task] Ignore isLoopRunning in MailboxExecutor.isIdle

When closing, this flag is set, but mailbox can still contain
(and receive new) mails (e.g. from timers) that should be processed.
In particular, this check currently prevents periodic watermarks from
being emitted.

flink-streaming-java/src/main/java/org/apache/flink/streaming/runtime/tasks/mailbox/MailboxExecutorImpl.java

@@ -57,8 +57,7 @@ public MailboxExecutorImpl(@Nonnull TaskMailbox mailbox, int priority, StreamTas
 	}
 
 	public boolean isIdle() {
-		return !mailboxProcessor.isMailboxLoopRunning() ||
-			(mailboxProcessor.isDefaultActionUnavailable() && !mailbox.hasMail() && mailbox.getState().isAcceptingMails());
+		return mailboxProcessor.isDefaultActionUnavailable() && !mailbox.hasMail() && mailbox.getState().isAcceptingMails();
 	}
 
 	@Override