setup: remove MANIFEST.in

Accoring to [1]

    […] More precisely, the following files are included in a source
    distribution by default:
    […]
    - All files specified by the license-files configuration parameter in
    pyproject.toml and/or equivalent in setup.cfg/setup.py; note that if you
    don’t explicitly set this parameter, setuptools will include any files that
    match the following glob patterns: LICEN[CS]E*, COPYING*, NOTICE*,
    AUTHORS**;
    - README, README.txt, README.rst or README.md;
    […]

Rendering this file unnecessary.

[1] https://setuptools.pypa.io/en/latest/userguide/miscellaneous.html

Author: behrmann