Fix unused imports

https://build.spring.io/browse/INT-MASTER-978/

**Cherry-pick to 5.0.x**

(cherry picked from commit 4f616e9)

Author: artembilan

spring-integration-test/src/test/java/org/springframework/integration/test/mock/MockMessageHandlerTests.java

@@ -49,7 +49,6 @@
 import org.springframework.integration.endpoint.EventDrivenConsumer;
 import org.springframework.integration.expression.ValueExpression;
 import org.springframework.integration.handler.ExpressionEvaluatingMessageHandler;
-import org.springframework.integration.handler.ServiceActivatingHandler;
 import org.springframework.integration.support.MessageBuilder;
 import org.springframework.integration.test.context.MockIntegrationContext;
 import org.springframework.integration.test.context.SpringIntegrationTest;
@@ -292,7 +291,7 @@ public ArgumentCaptor<Message<?>> argumentCaptorForOutputTest() {
 		@ServiceActivator(inputChannel = "nextChannel")
 		public MessageHandler handleNextInput() {
 			return mockMessageHandler(argumentCaptorForOutputTest())
-					.handleNext(m -> {});
+					.handleNext(m -> { });
 		}
 
 	}