If you look at your profile and look at time online, you'll see that it mostly just makes the screen a lot bigger and appears as unprofessional, to my thought at least. I think it's something which gives another a reason to only call you a "no life" or a term along those lines and not much else. This should be taken down as it would primarily improve our profile's appearance.
Online time is something nice to have. I see no good reason to remove it. If people don't think you have a life, that's their problem. We shouldn't completely get rid of one of the most important things on you're profile because of what people might say about you. If you're scared of people calling you a no-life, you're in the wrong place.
It can easily be removed in the templates, though I don't see why you want this removed? It shows how much time you have shown over the months/years on the website. It's a stat, like any other. You shouldn't be ashamed of what you have accomplished with anything, online included.
The ONLY reason you would want this removed is because you actually give two shits what others think of you which in the long wrong ruins most opportunities in life for you. I say keep it as it is.