fix Trigger references in the stream cookbook example for digest #24317

* replace Trigger with Unit
 * cosmetic changes in docs.stream.cookbook tests

akka-docs/src/test/java/jdocs/stream/javadsl/cookbook/RecipeTest.java

@@ -33,10 +33,4 @@ public abstract class RecipeTest extends AbstractJavaTest {
       return msg != null ? msg.hashCode() : 0;
     }
   }
-
-  final class Trigger {
-  }
-
-  final class Job {
-  }
 }