TYP only set default for one overloaded function in reset_index
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them

@simonjayhawkins I think we only need `=...` in one of the two overloads, then the error goes away and all passes:
```console
$ mypy pandas
Success: no issues found in 1244 source files
```