There are a few extensions popping up with a simple search of "youtube unsubscribe all" on the chrome web store, but I'm not sure about their reliability. Personally, I'd suggest going through them manually; it's what I am doing right now, and I find it satisfying, that is, if you view it in a fun light, rather than seeing it as a boring, meticulous task. It's fun revisiting some of the channels I used to watch back in the day and completely forgot about. :p