After elastic#63811 it became clear to me that `postCollect` is kind of
dangerous and not all that useful. So this removes it.

The trouble with `postCollect` is that it all happened right after we
finished calling `collect` on the `LeafBucketCollectors` but before we
built the aggregation results. But in elastic#63811 we found out that we can't
call `postCollect` on the children of `parent` or `child` aggregators
until we know which *which* aggregation results we're building.

So this removes `postCollect` and moves all of the things we did at
post-collect phase into `buildAggregations` or into hooks called in
those methods.