Remove deprecated license_file from setup.cfg

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

And `LICENSE` is still included in sdists thanks to setuptools-scm:

* pytest-dev#6348 (comment)

setup.cfg

@@ -10,7 +10,6 @@ project_urls =
 author = Holger Krekel, Bruno Oliveira, Ronny Pfannschmidt, Floris Bruynooghe, Brianna Laugher, Florian Bruhin and others
 
 license = MIT license
-license_file = LICENSE
 keywords = test, unittest
 classifiers =
     Development Status :: 6 - Mature