Remove content pages AB test

[sihugh]

The test is now complete, so we can remove it.

https://trello.com/c/ur3nYBPJ/166-to-remove-our-content-pages-experiment-from-the-a-b-framework

This removes the AB test, the "grey box" breadcrumb, the amendments to the sidebar and the extra navigation section at the bottom (including supertypes and topics)

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Example page:
https://government-frontend-pr-1088.herokuapp.com/contact-student-finance-england

[vanitabarrett]

@sihugh Can we also delete the following?

• https://github.com/alphagov/government-frontend/blob/master/config/initializers/logstasher.rb
• https://github.com/alphagov/government-frontend/blob/master/test/support/rummager_helpers.rb
• https://github.com/alphagov/government-frontend/blob/master/app/services/rummager_fields.rb
• https://github.com/alphagov/government-frontend/blob/master/config/locales/en.yml#L53