Doesn't exactly sound like a thing to be that concerned about if you never plug in your TV. If you buy a TV right now and never plug it in, how would it know about any future deal with Xfinity to connect to its open networks? And if it had a feature to connect to random open Wi-Fi networks right now, that wouldn't be hard to prove. My LG has never shown up in my Wi-Fi client list despite my open guest Wi-Fi. You can even test it by naming your guest network "Xfinity WiFi".

Taking apart your TV and pulling out the wireless radio sounds like absolute overkill to me. Feel free to do it if it makes you feel better. It will definitely not allow it to send data (over Wi-Fi) under any circumstance. I'm just not convinced that the alternative is that much riskier.