Remove unused import from ErrorMessageExceptionTypeRouter

**Auto-cherry-pick to `7.0.x`**

Author: artembilan

spring-integration-core/src/main/java/org/springframework/integration/router/ErrorMessageExceptionTypeRouter.java

@@ -28,7 +28,6 @@
 import org.springframework.jmx.export.annotation.ManagedAttribute;
 import org.springframework.jmx.export.annotation.ManagedOperation;
 import org.springframework.messaging.Message;
-import org.springframework.util.Assert;
 import org.springframework.util.ClassUtils;
 
 /**