fix: remove flake8 configuration file

[jorgepiloto]

This pull-request removes the configuration file for .flake8. We are fully supporting Ruff, so it makes no sense to keep this file around.

[github-actions]

The towncrier template has been successfully rewritten with the default template.
Please ensure that sphinx-design is included as a dependency in your project's documentation requirements.
Additionally, add sphinx-design to the extensions list in your conf.py file.
For more information, please refer to the migration guide at:
https://actions.docs.ansys.com/version/stable/migrations/index.html#migration-guide

[MaxJPRey]

LGTM.