Remove tooling related to using git

[pekkaklarck]

There are various Git workflows and updating our tooling to support them all is a big task. Much easier to list those Git commands in BUILD.rst and let users configure them if needed. Additionally, there's not much benefits in using

invoke set-version dev --push

compared to

invoke set-version dev
git commit -m "Back to dev version" rellu/__init__.py
git push

when all these commands anyway are listted on BUILD.rst.