Remove unused import

https://build.spring.io/browse/INT-SI43X-298/

Author: artembilan

spring-integration-core/src/test/java/org/springframework/integration/gateway/GatewayInterfaceTests.java

@@ -72,7 +72,6 @@
 import org.springframework.integration.context.IntegrationProperties;
 import org.springframework.integration.handler.AbstractReplyProducingMessageHandler;
 import org.springframework.integration.handler.BridgeHandler;
-import org.springframework.integration.handler.DelayHandler;
 import org.springframework.integration.support.MessageBuilder;
 import org.springframework.integration.test.util.TestUtils;
 import org.springframework.messaging.Message;