CLN: remove redundant openpyxl type conversions
openpyxl already converts cell values to Python types, preferring
integers where possible. For backwards-compatibility, we have to convert
integers back to float if not `convert_float`

- [ ] closes #xxxx
- [x] tests passed
- [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
- [ ] whatsnew entry
