Remove db/fixtures/test/01_admin.rb (EE backport)

What does this MR do?

We realized in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/3593 that introducing db/fixtures/test/01_admin.rb caused more harm than good. We removed this file in EE, so we should remove it in CE as well for now.

Improvements to the seeder (https://gitlab.com/gitlab-org/gitlab-ce/issues/28149), and migration paths (https://gitlab.com/gitlab-org/gitlab-ce/issues/40789) jobs are already tracked.

Why was this MR needed?

Because the previous fix from https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/15661 broke EE since we're seeding before(:all) there. It also introduced another issue that has been fixed (https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/15690), so the change wasn't that useless after all, it made our code more robust. ;)

Does this MR meet the acceptance criteria?

• Review
  • Has been reviewed by Backend
• Conform by the style guides
• Squashed related commits together