WIP: Pull mirroring and CI/CD for external repositories are no longer available on private projects without a subscription

What does this MR do?

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/10361 by reverting https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/5036.

• Put behind feature flag?
• Find affected users and post in https://gitlab.com/gitlab-org/gitlab-ee/issues/10361

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• [-] Documentation reviewed by technical writer or follow-up review issue created
• [-] Tests added for this feature/bug
• [-] Tested in all supported browsers
• Conforms to the code review guidelines
• [-] Conforms to the merge request performance guidelines
• [-] Conforms to the style guides
• [-] Conforms to the database guides
• [-] Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• [-] EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?
• [-] Security reports checked/validated by reviewer