Remove ineffective description-file key from setup.cfg

Otherwise, setuptools warns that:
"UserWarning: Usage of dash-separated 'description-file' will not be
supported in future versions. Please use the underscore name
'description_file' instead"

This key doesn't seem to do anything unless you use pbr.

setup.cfg

@@ -5,6 +5,4 @@ zip_safe=
 universal = 1
 
 [metadata]
-description-file = README.md
 license_file = LICENSE
-