ci: remove obsolete Blockscout image workflows

[JOY (JOY)]

Summary

• remove the unused Deploy from main branch workflow
• remove its now-unreferenced legacy Blockscout GHCR publisher
• keep the active DOS publisher for GHCR and Docker Hub unchanged

Evidence

• both legacy workflows have zero recorded runs
• the legacy publisher targets ghcr.io/blockscout/frontend-private or ghcr.io/blockscout/frontend and requires the Blockscout build runner plus ARM runner secrets
• no caller exists outside deploy-main.yml in this repo, the DOS organization, or public GitHub code
• the DOS publisher has 21 runs and successfully published v2.10.0 to ghcr.io/dos/doscan-frontend and metados/blockscout-frontend
• Docker Hub latest resolves to a published OCI manifest

Validation

• git diff --check origin/main...HEAD
• no remaining deploy-main.yml, publish-image.yml, Deploy from main branch, or Blockscout private frontend publisher references in automation paths
• full actionlint now reports only pre-existing findings in checks.yml and stale-issues.yml