Remove PushTopNToSource support for ExchangeExec (elastic#114637)

This appears to be dead code, so we're removing it.

Author: craigtaverner

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/optimizer/rules/physical/local/PushTopNToSource.java

@@ -24,7 +24,6 @@
 import org.elasticsearch.xpack.esql.optimizer.PhysicalOptimizerRules;
 import org.elasticsearch.xpack.esql.plan.physical.EsQueryExec;
 import org.elasticsearch.xpack.esql.plan.physical.EvalExec;
-import org.elasticsearch.xpack.esql.plan.physical.ExchangeExec;
 import org.elasticsearch.xpack.esql.plan.physical.PhysicalPlan;
 import org.elasticsearch.xpack.esql.plan.physical.TopNExec;
 
@@ -82,17 +81,6 @@ public PhysicalPlan rewrite(TopNExec topNExec) {
         }
     }
 
-    /**
-     * TODO: Consider deleting this case entirely. We do not know if this is ever hit.
-     */
-    record PushableExchangeExec(ExchangeExec exchangeExec, EsQueryExec queryExec) implements Pushable {
-        public PhysicalPlan rewrite(TopNExec topNExec) {
-            var sorts = buildFieldSorts(topNExec.order());
-            var limit = topNExec.limit();
-            return exchangeExec.replaceChild(queryExec.withSorts(sorts).withLimit(limit));
-        }
-    }
-
     record PushableQueryExec(EsQueryExec queryExec) implements Pushable {
         public PhysicalPlan rewrite(TopNExec topNExec) {
             var sorts = buildFieldSorts(topNExec.order());
@@ -141,13 +129,6 @@ && canPushDownOrders(topNExec.order(), hasIdenticalDelegate)) {
             // With the simplest case of `FROM index | SORT ...` we only allow pushing down if the sort is on a field
             return new PushableQueryExec(queryExec);
         }
-        if (child instanceof ExchangeExec exchangeExec
-            && exchangeExec.child() instanceof EsQueryExec queryExec
-            && queryExec.canPushSorts()
-            && canPushDownOrders(topNExec.order(), hasIdenticalDelegate)) {
-            // When we have an exchange between the FROM and the SORT, we also only allow pushing down if the sort is on a field
-            return new PushableExchangeExec(exchangeExec, queryExec);
-        }
         if (child instanceof EvalExec evalExec && evalExec.child() instanceof EsQueryExec queryExec && queryExec.canPushSorts()) {
             // When we have an EVAL between the FROM and the SORT, we consider pushing down if the sort is on a field and/or
             // a distance function defined in the EVAL. We also move the EVAL to after the SORT.