[Java] Further simplify MoreFutures usage in case it is cause of issues with ForkJoin pool stuckness

[scwhittle]

With 2.63 we still observed a pipeline stuck in WriteShardsIntoTempFilesFn joining with no apparent activity on the ForkJoinPool.

Thus #29926 may still exist, perhaps just less common. This change further simplifies the futures constructed since we don't require the list and perhaps there is some edge case on joining a future contributing the allof future within a chained apply. We don't need the returned list in this case so can delegate just to CompletableFutures.allof alone.

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[scwhittle]

R: @Abacn
since you reviewed #33042

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