I deleted a less file which I don't need anymore. I also removed the entry from default_head_blocks.xml so that it does not load anymore.
I searched my whole theme for main.css and removed all references to it.
But if I execute
grunt refresh then I get Unable to resolve the source file for frontend/company/base/de_DE/css/main.less
So I executed
grunt clean && grunt refresh but it makes no difference.
I also flushed all caches.