untrackClass = (classListRef, poll, classes) => {
  classes.forEach(className => {
    decrementReference(poll, className);
    poll[className] === 0 && classListRef.remove(className);
  });
}
Untrack a class and remove from the given class list if the reference
reaches 0.
@param {Object} classListRef A class list of an element.
@param {Object} poll         The poll to be used.
@param {Array}  classes      The list of classes to be untracked.