Ignore `stderr` when running `rofi`

[simonwiles]

Today after some system upgrades rofi on my system is outputting a warning on stderr[^*] which causes rofi-code to fail to open workspaces correctly. Digging in to the code a little, it seems that it's the use of CombinedOutput that's causing the problem. I think there's no reason why stderr should ever be passed to codeCmd?

Thanks!

[^*]: Fontconfig warning: using without calling FcInit() specifically, but that's not the point.