Remove aggregation's postCollect phase (#64016)

[nik9000]

After #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 #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.