DOC: scripts/validate_docstrings.py should clean up files created from docstrings
Discovered in https://github.com/pandas-dev/pandas/pull/24194

 Running 

`$ python scripts/validate_docstrings.py pandas.read_excel`

Produces a file `tmp.xlsx` in the pandas directory from a docstring in read_excel. We don't want to be accidentally committing these files; therefore, `validate_docstrings.py` should be removing any temporary artifacts after runs.

cc @datapythonista 