Revert "CI: run untrusted and GitHub-hosted Linux jobs on Blacksmith's Ubuntu 24.04 runners (#2237)" (#2277)

Blacksmith isn't provisioning runners: its dashboard says the account
"requires additional verification", so every job labelled blacksmith-*
stays queued with no runner. Fork PRs' Linux jobs go back to the
self-hosted-untrusted pool (#2249/#2252) and the rest to GitHub's runners
until Blacksmith verifies the account; then #2237 can be re-applied.

This reverts commit dc40f051b1.


Claude-Session: https://claude.ai/code/session_01LGeCa5jWWdroMWvXxN3LLM

Co-authored-by: jyviko <2501648+jyviko@users.noreply.github.com>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

.github/actionlint.yaml

@@ -2,8 +2,3 @@ self-hosted-runner:
   labels:
     - self-hosted-fast
     - self-hosted-light
-    # Blacksmith (Ubuntu 24.04 only; see the top of workflows/ci.yml).
-    - blacksmith-2vcpu-ubuntu-2404
-    - blacksmith-4vcpu-ubuntu-2404
-    - blacksmith-8vcpu-ubuntu-2404
-    - blacksmith-16vcpu-ubuntu-2404