Move code for deleting old cookies into banner JS

Removes the following cookies:
- seen_cookie_message (flags if banner was already
  shown)
- _gid (Google Analytics cookie)
- _ga (Google Analytics cookie)

These were set by default before so potentially
still around for some users.

The code for this now exists as a static method on
the cookieMessage module and is called when the JS
loads for the first time.

app/assets/javascripts/main.js

@@ -1,6 +1,6 @@
 window.GOVUK.Frontend.initAll();
 
-$(() => GOVUK.addCookieMessage());
+window.GOVUK.Modules.CookieBanner.clearOldCookies();
 
 if (window.GOVUK.hasConsentFor('analytics')) {
   window.GOVUK.initAnalytics();