Remove references to the Content API
Remove references to the Content API

The [`content_api`](https://github.com/alphagov/govuk_content_api) has
been retired, and all content has been migrated to the
[`content_store`](https://github.com/alphagov/content-store).

Therefore, we can remove all references to the Content API, and rely
solely on the Content Store.

alphagov/frontend,alphagov/frontend,alphagov/frontend,alphagov/frontend