Cleanup BucketsAggregator#rewriteBuckets (elastic#114574)

The array is initialized with the flag clearOnResize set to true so we don't need to set the values to 0 again.

server/src/main/java/org/elasticsearch/search/aggregations/bucket/BucketsAggregator.java

@@ -105,7 +105,6 @@ public final void rewriteBuckets(long newNumBuckets, LongUnaryOperator mergeMap)
         try {
             docCounts = bigArrays().newLongArray(newNumBuckets, true);
             success = true;
-            docCounts.fill(0, newNumBuckets, 0);
             for (long i = 0; i < oldDocCounts.size(); i++) {
                 long docCount = oldDocCounts.get(i);