ESQL: Simplify agg construction when reducing on node-level (elastic#128980) (elastic#129050)

(cherry picked from commit 110bbe8)

# Conflicts:
#	x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/planner/AbstractPhysicalOperationProviders.java

Author: alex-spies

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/planner/AbstractPhysicalOperationProviders.java

@@ -33,7 +33,6 @@
 import org.elasticsearch.xpack.esql.expression.function.aggregate.Count;
 import org.elasticsearch.xpack.esql.expression.function.grouping.Categorize;
 import org.elasticsearch.xpack.esql.plan.physical.AggregateExec;
-import org.elasticsearch.xpack.esql.plan.physical.ExchangeSourceExec;
 import org.elasticsearch.xpack.esql.planner.LocalExecutionPlanner.LocalExecutionPlannerContext;
 import org.elasticsearch.xpack.esql.planner.LocalExecutionPlanner.PhysicalOperation;
 
@@ -70,14 +69,6 @@ public final PhysicalOperation groupingPhysicalOperation(
 
         var sourceLayout = source.layout;
 
-        if (aggregatorMode != AggregatorMode.INITIAL && aggregatorMode != AggregatorMode.FINAL) {
-            assert false : "Invalid aggregator mode [" + aggregatorMode + "]";
-        }
-        if (aggregatorMode == AggregatorMode.INITIAL && aggregateExec.child() instanceof ExchangeSourceExec) {
-            // the reducer step at data node (local) level
-            aggregatorMode = AggregatorMode.INTERMEDIATE;
-        }
-
         if (aggregateExec.groupings().isEmpty()) {
             // not grouping
             List<Aggregator.Factory> aggregatorFactories = new ArrayList<>();

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/planner/PlannerUtils.java

@@ -99,7 +99,7 @@ public static PhysicalPlan reductionPlan(PhysicalPlan plan) {
         final LocalMapper mapper = new LocalMapper();
         PhysicalPlan reducePlan = mapper.map(pipelineBreaker);
         if (reducePlan instanceof AggregateExec agg) {
-            reducePlan = agg.withMode(AggregatorMode.INITIAL); // force to emit intermediate outputs
+            reducePlan = agg.withMode(AggregatorMode.INTERMEDIATE);
         }
         return EstimatesRowSize.estimateRowSize(fragment.estimatedRowSize(), reducePlan);
     }