Remove ineffective description-file parameter from setup.cfg

Otherwise, we get a warning:
"Usage of dash-separated 'description-file' will not be supported in
future versions."

This parameter was only used when setup.py was used with pbr=True, which is
not the case here, and setup.py loads the right file into long_description.

setup.cfg

@@ -1,2 +0,0 @@
-[metadata]
-description-file = README.md