Revert "Label list shows all issues (opened or closed) with that label"

What does this MR do?

Revert changes made on https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5991. Reasoning here: https://gitlab.com/gitlab-org/gitlab-ce/issues/21691#note_16516861

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #21691 (closed)