If you asked me, I would only take down an old work of mine if it contained overly offensive moments or subtexts that I wouldn't want to be associated with me. Maybe those things flew over my head when I was writing it or I was just a different person then compared to now.
That, or if it was just really, really embarrassing (like stuff I would write when I was 12 and think it was "cool") and potentially turn away whomever looked at it.
If your series is just sitting there, then I'd suggest keeping it up. It's proof of your growth as an artist/writer which is a nice thing to look at every now and again.
In the end, it's all up to you really. No other way of putting that. But I will say that if I'm looking through the works of an online artist I admire, I'd always want to check some of their early works out of curiosity, to see what else they've made. Regardless of quality, I'm sure there's some entertainment to be found and who knows, it might even be pretty good.
So, if you have fans like that, it might be worth keeping it up.