Remove save on signout code

[ajmaddaford]

What is the context of this PR?

Removes redundant save on signout code and tests. This hasn't been used since the signout button was changed to a link in ONSdigital/eq-survey-runner#2277.

As part of this work a new signed-out endpoint has been added to differentiate it from sign-out. sign-out does the work of signing out and redirecting to either a provided log out url or the signed-out page.

The sign out button logic has been simplified to remove the check for an account_service_log_out_url. This was to hide the sign out button on the thank-you page. Instead there is now an explicit property hide_signout_button and cookie state modification on the GET of the thank-you page has been removed.

How to review

• Check that all code related to save on signout has been removed
• Check that tests that have been removed should have been

Checklist

• New static content marked up for translation
• Newly defined schema content included in eq-translations repo

[LJBabbage]

Looks good to me, bar the odd comment in the tests