Remove support for sorting terms aggregation by ascending count

[colings86]

We try to be as flexible as possible when it comes to sorting terms aggregations. However, sorting by anything but by _term or descending _count makes it very hard to return the correct top buckets and counts, which is disappointing to users. For this reason we should remove the ability to sort the terms aggregation by ascending count (split out from #17588)

[rashidkpc]

If and when you decide if you're going to do this, please open an issue on kibana. We currently support this in the UI so we'll need to issue a deprecation notice well in advance

[rashidkpc]

Oh, also, I'm +1 on this, its confusing and rarely useful anyway. If we're going to deprecate it pre-5.0.0, which I'd prefer, let me know.