Remote assertion that relies on all shards being successful

The assertion that if there are buffered aggs at least one incremental
reduce phase should have happened doens't hold if there are shard failure.
This commit removes this assertion.

Relates to #23288

Author: s1monw

core/src/main/java/org/elasticsearch/action/search/SearchPhaseController.java

@@ -500,7 +500,6 @@ private ReducedQueryPhase reducedQueryPhase(List<? extends AtomicArray.Entry<? e
         final List<InternalAggregations> aggregationsList;
         if (bufferdAggs != null) {
             consumeAggs = false;
-            assert numReducePhases > 1 : "num reduce phases must be > 1 but was: " + numReducePhases;
             // we already have results from intermediate reduces and just need to perform the final reduce
             assert firstResult.hasAggs() : "firstResult has no aggs but we got non null buffered aggs?";
             aggregationsList = bufferdAggs;