chore: remove unused deprecated polyfill.js file (#6345)

* config(dev): remove unused polyfills in webpack-dev

src/polyfills.js

@@ -1,12 +0,0 @@
-// Promise global, Used ( at least ) by 'whatwg-fetch'. And required by IE 11
-
-import win from "core/window"
-
-if(typeof win.Promise === "undefined") {
-  require("core-js/fn/promise")
-}
-
-// Required by IE 11
-if(!String.prototype.startsWith) {
-  require("core-js/es6/string")
-}