To remove the Google fonts all together, you can look in your functions.php file and remove the code from there.
For your last question I’m not sure if that will work or not. You can test it to see after you remove the Google font code from your functions file. If you’re saying that Browsers will show Open Sans as one of their fonts without needing to link to Googles Font servers, then I guess that would work. I’m not sure though, so more research would need to be done to see if that’s true/possible.