Removed Admin production build test
Removed Admin production build test

- this was originally added because we had an issue with a production
  build of Admin during a release
- since then, we've got a canary build that runs for every commit on
  `main`
- I think this test is superfluous now, so this commit removes it and
  saves ~6 mins of CI time per commit

TryGhost/Ghost,TryGhost/Ghost,TryGhost/Ghost