TYP, CLN annotate some variables, remove some possibly unused ones
- [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

The [mypy daemon](https://mypy.readthedocs.io/en/latest/mypy_daemon.html) throws some extra `var-annotated` errors compared with `mypy pandas`, so I've tried fixing some. Some of the variables it complains about seem to be unused (ExcelWriter.curr_sheet and NDFrame._ix) - are these safe to remove?