setup.cfg: remove deprecated "MIT License" classifier

This was pointed out by the `build` job in the `release.yml` workflow:

```
/tmp/build-env-b5h2pv2g/lib/python3.13/site-packages/setuptools/dist.py:759: SetuptoolsDeprecationWarning: License classifiers are deprecated.
!!

        ********************************************************************************
        Please consider removing the following classifiers in favor of a SPDX license expression:

        License :: OSI Approved :: MIT License

        See https://packaging.python.org/en/latest/guides/writing-pyproject-toml/#license for details.
        ********************************************************************************

!!
```

Author: generalmimon

setup.cfg

@@ -18,7 +18,6 @@ classifiers =
     Development Status :: 4 - Beta
     Intended Audience :: Developers
     Topic :: Software Development :: Build Tools
-    License :: OSI Approved :: MIT License
     Programming Language :: Python :: 2
     Programming Language :: Python :: 2.7
     Programming Language :: Python :: 3