Remove count() method from GeoCentroid aggregation interface

[cbuescher]

This change removes the count() method from the GeoCentroid interface used for the geo_centroid aggregation. It seems to be unused at least in our own codebase and we also don't write the count value to the aggregation Rest response but only use it in the reduce method.
This is the main reason for removing the method from the interface. When adding parsing for the aggregation responses for the
Highl level Rest Client we strive to provide all information available via the REST API to the user through the aggregation interface and it only makes sense to provide values through the interface that are also provided via REST.
The method could be deprecated in 5.x if necessary.

[cbuescher]

@colings86 assigning to you first again because you might have an opinion whether count() is still needed in the interface or not. Otherwise please reassign.

[cbuescher]

Also it looks like it's not possible to get the count value using the InternalGeoCentroid#getProperty() method.

[colings86]

I think we should keep count here since its useful to know how many values contributed to the centroid. We could add it to the getProperty() method but I don't see that as being an urgent thing

[cbuescher]

Then we should also add it to the REST response. Do you think we can do that in v6.0? Otherwise we have no way of parsing that value back on the client side.

[colings86]

Yes, I think it would be useful to add it to the REST response in v6.0. /cc @jpountz @polyfractal do you agree?

[polyfractal]

++ adding to response. Could definitely see this being used, e.g. on a chart, size the centroid marker based on number of values in the centroid.

[javanna]

If we add this field to the REST response, what's keeping us from adding it in 5.x as well? Adding a field should not be considered a breaking change should it?

[cbuescher]

I'll close this in favour of #24387