BUG: not dropping scalar-indexes MultiIndex levels
- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [ ] whatsnew entry

There are a bunch of issues about inconsistent dropping of MultiIndex levels: #12827, #18631, #10552, #42102, #41416, none of which are closed by this.