[IMP] kanban: removed / factorized custom css

- Task: https://www.odoo.com/web#id=8367&view_type=form&model=project.task&menu_id=3942&action=327
- Pad: https://pad.odoo.com/p/r.yL5gHFj7mUTj6Fb5
Sorry, my bad, it is defined in `website.sass`, so you can keep your code