BUG: sort_index did not respect ignore_index when not sorting
- [X] closes #43591
- [X] tests added / passed
- [X] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit) for how to run them
- [X] whatsnew entry

thanks @saehuihwang very nice!