I wonder whether I only missed the feature because it seems pretty obvious to me. So the way I see it, it's possible to uninstall games through the itch.io client, but it's not possible to remove games from my collection, which is unfortunate.
I usually browse the website for games I would like to try, add them to the collection and play them later. Now when I am playing, I'm not using the website at all, so if I realize that I don't like the game very much, I would want to remove it from my collection, but it seems I can't. I have to go to the website to do that.
If it is possible to do so, maybe it should be made clearer how (or I am just too dense).