TST: use ensure_clean rather than explicit os.remove
```
~/pandas-dev$ grep -r os.remove pandas/tests/
pandas/tests/io/excel/test_openpyxl.py:    os.remove(filename)
pandas/tests/io/excel/test_writers.py:    #     os.remove(filename)
pandas/tests/io/excel/test_writers.py:    #     os.remove(filename)
pandas/tests/io/pytables/common.py:            os.remove(path)
```

we should also create a rule in code_checks.sh to avoid doing this in the future