Exclude polyfills for DOM collection iteration

Although these polyfills are required to iterate DOM collections, we always use `Array.from()` so don’t need them:

```
The corejs3 polyfill added the following polyfills:
  es.array.iterator { "chrome":"61", "opera":"48", "samsung":"8.2" }
  web.dom-collections.iterator { "chrome":"61", "edge":"16", "ios":"10.3", "opera":"48", "safari":"10.1", "samsung":"8.2" }
```

packages/govuk-frontend/babel.config.js

@@ -53,6 +53,10 @@ module.exports = function (api) {
             // https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Error/cause
             'es.error.cause',
 
+            // DOM collection iteration is unnecessary with `Array.from()`
+            // see https://github.com/zloirock/core-js/issues/1003
+            'web.dom-collections.iterator',
+
             // WHATWG URL constructor `undefined` base fix is unnecessary
             // https://bugs.webkit.org/show_bug.cgi?id=216841
             // https://github.com/zloirock/core-js/commit/0ef4be7352e900dfa6f1eda27de22f7974576531