Removed unused import

spring-messaging/src/main/java/org/springframework/messaging/handler/AbstractMessageCondition.java

 /*
- * Copyright 2002-2018 the original author or authors.
+ * Copyright 2002-2019 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -17,7 +17,6 @@
 package org.springframework.messaging.handler;
 
 import java.util.Collection;
-import java.util.Iterator;
 import java.util.StringJoiner;
 
 import org.springframework.lang.Nullable;