Remove Function.bind polyfill

- Not used anymore, and phantomjs 2 has a native .bind

ghost/admin/app/utils/bind.js

@@ -1,13 +0,0 @@
-const {slice} = Array.prototype;
-
-export default function (/* func, args, thisArg */) {
-    let args = slice.call(arguments);
-    let func = args.shift();
-    let thisArg = args.pop();
-
-    function bound() {
-        return func.apply(thisArg, args);
-    }
-
-    return bound;
-}