Remove :rbac_clusters feature flag

What does this MR do?

Now that Auto DevOps can run in a RBAC enabled cluster from #51942 (closed), we can now remove the FF.

As the flag only governed if UI elements would appear or not when creating/adding existing clusters; the effect of removing the FF would be that the checkbox to choose to enable RBAC clusters will now always appear.

TIP review this change with "Hide whitespaces" :)

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/51009

Testing

• package-and-qa (all passes except ce:update - known issue)
• Run Auto Devops pipeline for RBAC-only cluster
  • Run Feature.disable :rbac_clusters
  • restart gdk
  • create new RBAC-only cluster
  • run auto-devops pipeline

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides

Closes #51009 (closed)