create_listener = function(elem) {
    var listener = function() {
        elem.value = '';
        elem.removeEventListener('focus', listener, false);
    };
    return listener;
}

prepare_delete_onfocus = function() {
    var elems, elemI, elem, listener;
    elems = document.getElementsByClassName('deleteonfocus');
    for (elemI in elems) {
        elem = elems[elemI];
        elem.addEventListener('focus', create_listener(elem), false);
    }
}

window.onload = function(event) {prepare_delete_onfocus();}