Remove legacy nested menu items javascript and styling

[GuySartorelli]

There is some frankly broken styling with associated functionality for having nested CMS menu items.
This is in client/src/components/Menu/Menu.scss and client/src/legacy/LeftAndMain.Menu.js.
Adding nested CMS menu items isn't documented outside of a doc comment in the above noted js file.

Nested menu items are possible through a third part module which currently relies on this styling and javascript (which is the only reason I haven't removed it in CMS4).
This broken styling and dead js functionality should be removed - it can be reimplemented in the third-party module itself, since that's the only place I can find that uses it.

Acceptance criteria

• Remove the broken css and js

PRs

• MNT Remove unused JS and CSS #1818

[GuySartorelli]

PR merged