Remove CardinalityAggregatorColumnSelectorStrategy and CardinalityAggregatorColumnSelectorStrategyFactory as too shallow abstractions.

[leventov]

The logic of CardinalityAggregatorColumnSelectorStrategyFactory could be inlined in CardinalityAggregatorFactory.factorize() and factorizeBuffered() methods, and the logic of CardinalityAggregatorColumnSelectorStrategy could be expressed in subclasses of CardinalityBufferAggregator and CardinalityAggregator.

This change is similar to what is proposed here: #6397 (comment)

[hpandeycodeit]

@leventov I will work on this one.

[gianm]

I accidentally stumbled into this by suggesting the opposite refactoring on #7496: #7496 (comment). I think the way that CardinalityAggregator works now is pretty nice, for the reasons I mentioned in that thread. I'm not really interested in requiring that everything be done that way but I did want to speak up for why the design is the way it is.

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