@Klaas - It might be an issue of precedence. This works if I put it near the top of my stylesheet, but not when I put it in a snippet or at the bottom. I don’t have time to debug, but there’s something farther down that’s breaking it.
.workspace-ribbon.mod-left {width:0px;}
You could similarly add .mod-right to hide the other one, too.
If you want to reclaim the space it was occupying, include this:
.workspace-split.mod-left-split {margin-left: 0px;}