Remove unknown pytest.mark.unit

[geographika]

Fixes warning:

OWSLib/OWSLib/tests/test_iso3_parsing.py:48: PytestUnknownMarkWarning: Unknown pytest.mark.unit - is this a typo?  You can register custom marks to avoid this warning - for details, see https://docs.pytest.org/en/stable/how-to/mark.html

The custom unit mark isn't used anywhere in the project.