Remove deprecated license_file from setup.cfg (#825)

Starting with wheel 0.32.0 (2018-09-29), the "license_file" option is
deprecated.

https://wheel.readthedocs.io/en/stable/news.html

The wheel will continue to include LICENSE, it is now included
automatically:

https://wheel.readthedocs.io/en/stable/user_guide.html#including-license-files-in-the-generated-wheel-file