Remove pages_multiple_versions_enable user setting

What does this MR do and why?

This MR removes the user setting for enabling Pages parallel deployments (aka multiple versions). It was originally put in as a user-facing switch for an in-development feature. Since there is nothing that this setting protects, it is better UX to simply remove it.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Set up your GDK to support runners.
2. Create a new project from the "Pages/Plain HTML" template.
3. Update the .gitlab-ci.yml file and add the below to the pages job:

pages:
  path_prefix: foo

3. Ensure the pipeline runs correctly
4. go to "Deploy>Pages" and verify under "Settings" no longer is a checkbox for "Enable multiple deployments"

Related to #502219 (closed)