Use directiveId instead of activityParents to compute directive id

DirectiveId used to be computed from activityParents, but as of
recently it is computed separately. This commit uses it directly

merlin-driver/src/main/java/gov/nasa/jpl/aerie/merlin/driver/engine/SimulationEngine.java

@@ -828,7 +828,7 @@ public SimulationActivityExtract computeActivitySimulationResults(
                 .stream()
                 .map(spanToActivityInstanceId::get)
                 .toList(),
-            (activityParents.containsKey(span)) ? Optional.empty() : Optional.ofNullable(directiveId),
+            Optional.ofNullable(directiveId),
             outputAttributes
         ));
       } else {
@@ -843,7 +843,7 @@ public SimulationActivityExtract computeActivitySimulationResults(
                 .stream()
                 .map(spanToActivityInstanceId::get)
                 .toList(),
-            (activityParents.containsKey(span)) ? Optional.empty() : Optional.ofNullable(directiveId)
+            Optional.ofNullable(directiveId)
         ));
       }
     });