In the following code
document.cookie = allcookies + "=" + cval + "; expires=" + exp.toGMTString();
allcookies should be the name of the specific cookie that you are deleting. [You don't need to set the 'name=value', just 'name=;expires=santasdayout']
BTW the following only subtracts 1 millisecond, but it is immaterial
exp.setTime(exp.getTime() - 1); // set it to yesterday!
But you should also validate
your HTML as you are missing the html tag, and "text|hidden" is incorrect.
BTW I don't think 'button' is a sensible name for a button
.. I know it's a button!