Remove legacy design system implementations from the review app

[owenatgov]

What

Remove our legacy stylesheets which we use to mimic a service which uses legacy design system implementations.

Why

We are removing compatibility mode in 5.0 and therefore it makes sense to also remove this from our own review app as they are no longer relevant.

Part of #2769

Who needs to work on this

Developers

Done when

• Decide which legacy examples we should keep and move over to govuk-frontend properly and which examples we should remove
• Remove https://github.com/alphagov/govuk-frontend/blob/main/app/assets/scss/app-legacy-ie8.scss
• Remove https://github.com/alphagov/govuk-frontend/blob/main/app/assets/scss/app-legacy.scss
• Remove legacy implementations as npm dependancies