chore(ci): remove per-repo GHCR cleanup (centralized in actions) (#795)

Org-wide GHCR cleanup is now handled in projectbluefin/actions:
projectbluefin/actions/.github/workflows/ghcr-cleanup.yml

Covers: bluefin, bluefin-nvidia, bluefin-pr, dakota, dakota-nvidia, common

Closes projectbluefin/common#603

Assisted-by: Claude Sonnet 4.5 via pi

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: castrojo