Remove built govuk-frontend from committed files

[colinrotherham]

This PR removes the govuk-frontend build output we use for npm publish

This was known as the ./package directory until it moved to ./packages/govuk-frontend/dist in:

• Update Review app to import govuk-frontend via local package #3491

We don't need to commit it as we rebuild it during GitHub Actions tests

But it also addresses other concerns found during the performance work:

1. Our build outputs ESM files but we only test the UMD bundles
2. Our build outputs Sass with autoprefixer, but we use ./src instead
3. Our build outputs Nunjucks templates, but we use ./src instead
4. Our build outputs JSON component data, but we use ./src instead
5. Our build outputs font and image assets, but we use ./src instead
6. Review app ignores ./package and builds its own files from ./src

Point 1) has been resolved in #3491
Point 4) has been deemed acceptable for now

Once deleted, we can import from the built govuk-frontend instead

[romaricpascal]

Worked OK for dev (including watch), build:package as well as using npm link or a pre-release branch to use it in the Prototype Kit 🎉