You don't. There's folders full of textures, sounds, admin mod plugins, all kinds of shit. You start deleting those and some
may be replaced, but others will cause you to delete a bunch of maps and have it re-downloaded. If you see anything of the "Temp" or "Cache" variety, you can try, but I still wouldn't.
EDIT: I've used SteamTool to help me move games to other drives without ever re-installing, so it's super helpful, if you want to free up SSD. I put all my shit games like CS:GO on a shitty 250GB Western Digital.
Link:
http://www.stefanjones.ca/steam/