twitter-remove-char-limit.js

(function twitterRemoveCharLimit() {
    function hasClass(ele, cls) {
        return ele.className.match(new RegExp('(\\s|^)' + cls + '(\\s|$)'));
    }

    function removeClass(ele, cls) {
        if (hasClass(ele, cls)) {
            var reg = new RegExp('(\\s|^)' + cls + '(\\s|$)');
            ele.className = ele.className.replace(reg, ' ');
        }
    }
    var list = document.getElementsByClassName('tweet-action tweet-btn')
    for (var i = 0; i < list.length; i++) {
        removeClass(list[i], 'disabled')
        list[i].disabled = false
    }
})()

Uglyified with UglifyJS

function hasClass(s,e){return s.className.match(RegExp("(\\s|^)"+e+"(\\s|$)"))}function removeClass(s,e){if(hasClass(s,e)){var a=RegExp("(\\s|^)"+e+"(\\s|$)");s.className=s.className.replace(a," ")}}for(var list=document.getElementsByClassName("tweet-action tweet-btn"),i=0;i<list.length;i++)removeClass(list[i],"disabled"),list[i].disabled=!1

Bookmarklet

<a href='javascript:(function(){function hasClass(s,e){return s.className.match(RegExp("(\\s|^)"+e+"(\\s|$)"))}function removeClass(s,e){if(hasClass(s,e)){var a=RegExp("(\\s|^)"+e+"(\\s|$)");s.className=s.className.replace(a," ")}}for(var list=document.getElementsByClassName("tweet-action tweet-btn"),i=0;i<list.length;i++)removeClass(list[i],"disabled"),list[i].disabled=!1
})();'>Remove Tweet Char Limit</a>