Remove fetch option from ExternalSorter

[yyy1000]

Is your feature request related to a problem or challenge?

When I learn Sort implementation for #9170
I found now the fetch option in ExternalSorter is not used since #7721, see below
https://github.com/apache/arrow-datafusion/blob/497cb9d46c6f68de6762998c241d0860072c7909/datafusion/physical-plan/src/sorts/sort.rs#L868-L897
I wonder whether it's proper to remove this option to make it cleaner or keep it for possible future use.🤔

Describe the solution you'd like

remove the fetch option and related method

Describe alternatives you've considered

Keep it, maybe useful in future?

Additional context

I can help it if needed.

[alamb]

Maybe @kazuyukitanimura has some thoughts on this matter

[alamb]

I think removing the option seems to make sense to me if it is not used. We can always put it back

[yyy1000]

Thanks! @alamb
I'm also trying to use ExternalSorter in PartialSort to implement spill function. (Still working)
If I find the fetch option will not be used, I can remove it.

[kazuyukitanimura]

It is interesting. We do not have any tests for ExternalSorter.fetch, looks like? @yyy1000
We can remove it but since ExternalSorter.new(..., fetch, ...) is pub, I guess we should add a deprecation message first.