Remove pages_multiple_versions_enable user setting

Original discussion

From the main epic (&10914):

Add a kill switch for the site versioning: Add a pages_multiple_versions_enable to ProjectSetting, where the user can enable/disable the multiple version of GitLab Pages. This way the project owner has the power to disable the feature and avoid contributors to add multiple versions of the site.

It was a failsafe option from the early days of the feature. I'm still not sure every customer would like this option enabled by default as it might increase storage and delays on internal API. For this, and extra-security, reasons I created the option. I think removing it is a product decision. But yeah maybe enabling it by default might be a good middle-ground.

This setting introduces one more step before you can use the pages for multiple deployments feature making it harder to use.

Proposal

1. Either remove the setting completely if we're not sure it has value
2. At least make it enabled by default

We can also enable it by default and re-evaluate later, but it will be harder since it will technically be a breaking change.

As for the original point

avoid contributors to add multiple versions of the site

You can achieve that by protecting the ci file from change. So I don't think we need a duplicate feature.