[tox] Drop reference to removed init_instance.py file in flake8-ok-files.txt
The file got removed in 1b9fd8bda1cd.
[theme]
inherit = pyramid
pygments_style = sphinx.pygments_styles.PyramidStyle
stylesheet = cubicweb.css
[options]
logo = logo-cubicweb.svg
favicon = cubicweb.ico
in_progress = false
outdated = false
canonical_url =