turn off parallel writes in grid (#582)

Temporarily disabling parallel writes, awaiting more review and benchmarking.

Author: MarkWolters

jvector-examples/src/main/java/io/github/jbellis/jvector/example/Grid.java

@@ -308,8 +308,6 @@ private static BuilderWithSuppliers builderWithSuppliers(Set<FeatureId> features
         var builder = new OnDiskGraphIndexWriter.Builder(onHeapGraph, outPath);
         builder.withMapper(identityMapper);
 
-        // Enable parallel writes for improved throughput
-        builder.withParallelWrites(true);
         Map<FeatureId, IntFunction<Feature.State>> suppliers = new EnumMap<>(FeatureId.class);
         for (var featureId : features) {
             switch (featureId) {