Cleanup pages_multiple_versions_setting feature flag

What does this MR do and why?

This MR removes all references to the pages_multiple_versions_setting feature flag since it's been enabled by default on production.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

How to set up and validate locally

1. Please make sure that you have a GitLab Ultimate license in your GDK environment
2. Start on master branch, go to the console rails c and disable this feature by running Feature.disable(:pages_multiple_versions_setting)
3. Create "Create from template" project at http://gdk.test:3000/projects/new
4. Choose Pages/Plain HTML template
5. After the project is created, update the .gitlab-ci.yml file with the following:

image: busybox

pages:
  stage: deploy
  script:
    - echo "Pages site is deployed"
  artifacts:
    paths:
      - public
  pages:
    path_prefix: "v1"
  rules:
    - if: $CI_COMMIT_BRANCH == $CI_DEFAULT_BRANCH

6. The deploy should fail since the path_prefix is a parallel deployment feature that is behind the pages_multiple_versions_setting flag and we disabled it.
7. Checkout to this MR branch and retry the same failed pipeline, it should now succeed.

Related to #487161 (closed)