INT-1339: removed mark message to keep MessageGroup interface immutable

spring-integration-core/src/main/java/org/springframework/integration/store/MessageGroup.java

@@ -58,9 +58,4 @@ public interface MessageGroup {
 	 * @return the timestamp (milliseconds since epoch) associated with the creation of this group
 	 */
 	long getTimestamp();
-
-	/**
-	 * Mark the given message in this group. If the message is not part of this group then this call has no effect.
-	 */
-	void mark(Message<?> messageToMark);
 }

spring-integration-core/src/main/java/org/springframework/integration/store/SimpleMessageGroup.java

@@ -155,7 +155,7 @@ public class SimpleMessageGroup implements MessageGroup {
 	}
 
 	/**
-	 * {@inheritDoc}}
+	 * Mark the given message in this group. If the message is not part of this group then this call has no effect.
 	 */
 	public void mark(Message<?> messageToMark) {
 		synchronized (lock) {