The quickest method would be /topic # Ctrl-O
Granted, it's not *really* unset. But it may be your best option. You can use any ctrl character if you want, but if you use something other than Ctrl-O, then you could run into an issue if you did something like:
//echo -a $chan(#channel).topic is the topic
If you had Ctrl-B, you'd have bold text, just as an example.
If there is a real way to unset the topic, I don't know of it.