Delete unused imports in spring-messaging module

Author: sbrannen

spring-messaging/src/main/java/org/springframework/messaging/simp/config/AbstractMessageBrokerConfiguration.java

@@ -26,8 +26,6 @@
 import org.springframework.context.ApplicationContextAware;
 import org.springframework.context.annotation.Bean;
 import org.springframework.messaging.Message;
-import org.springframework.messaging.MessageChannel;
-import org.springframework.messaging.SubscribableChannel;
 import org.springframework.messaging.converter.*;
 import org.springframework.messaging.handler.invocation.HandlerMethodArgumentResolver;
 import org.springframework.messaging.handler.invocation.HandlerMethodReturnValueHandler;