Removes pipeline aggregations from the aggregation result tree as they
are no longer used. This stops us from building the pipeline aggregators
at all on data nodes except for backwards compatibility serialization.
This will save a tiny bit of space in the aggregation tree which is
lovely, but the biggest benefit is that it is a step towards simplifying
pipeline aggregators.
This only does about half of the work to remove the pipeline aggs from
the tree. Removing all of it would, well, double the size of the change
and make it harder to review.
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