Remove fetch option from ExternalSorter

### 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.

Start in datafusion/physical-plan/src/sorts/sort.rs at the linked ExternalSorter section and search the repository for fetch option and related method usages. Confirm the option is unused, remove it and its related method, then verify the project still builds and its existing tests pass.

Written by the indexing model from the issue text.