doc: remove black §, covered into pre-commit §

CONTRIBUTING.md

@@ -63,18 +63,6 @@ set -xg GITLAB_PRIVATE_TOKEN ""
 
 Check out #169 for more details.
 
-### Black formatting
-
-We are using [Black](https://github.com/psf/black) formatter tool.
-Run Black on a Python file to format it:
-
-```bash
-poetry run black duniterpy/file.py
-```
-
-With `pre-commit`, Black is called on staged files, so the commit should fail in case black would make changes.
-You will have to add Black changes in order to commit your changes.
-
 ## Tests
 
 We are using [`pytest` framework](https://docs.pytest.org/).