setup.py: drop data_files, installs LICENSE to incorrect place

[tpetazzoni]

As reported in issue https://github.com/xmlrunner/unittest-xml-reporting/issues/284, the data_files statement in the setup() calls installs the LICENSE file in the wrong place: in /usr/LICENSE, or even /LICENSE depending on the configuration.

So let's drop this, and let setuptools install the LICENSE file automatically: since setuptools v56, the license_files attribute is automatically assigned to a default value, which includes "LICENSE", so there is in fact nothing to do to the get the LICENSE file installed at the correct location. See https://setuptools.pypa.io/en/latest/history.html#v56-0-0.