[Bug]: Possibly unnecessary prefetch during GroupIntoBatches

### What happened?

As I was inspecting why the Dataflow "Total streaming data processed" metric was so much higher (2-4× depending on the pipeline) than the actual data that was being processed as it only has one shuffle/gbk/gib/etc transform I stumbled upon something that I couldn't justify being there:
https://github.com/apache/beam/blob/f549fd33abdc672143ccbe3f0f66104995d30fe6/sdks/java/core/src/main/java/org/apache/beam/sdk/transforms/GroupIntoBatches.java#L566-L569

I might be missing some context here, but I'm not even sure what benefit this provides for any generic `GroupIntoBatches` transform, but it seems to be especially useless if called on a `GroupIntoBatches.WithShardedKey` (when every data the batch contains comes from the same worker already).

I would appreciate any insight before I submit a PR to change this.

### Issue Priority

Priority: 3 (minor)

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