[VULN-649] ci: Remove deprecated scan workflow

[theMickster]

🎟️ Tracking

https://bitwarden.atlassian.net/browse/VULN-649

📔 Objective

Removes the deprecated scan.yml GitHub Actions workflow.

[github-actions]

🤖 Bitwarden Claude Code Review

Overall Assessment: APPROVE

This PR deletes the deprecated .github/workflows/scan.yml GitHub Actions workflow (the Checkmarx SAST and check-run jobs), tracked under VULN-649. The removal is complete — no remaining workflow, script, or config references the deleted scan.yml or its check-run/sast jobs. The remaining scan.yml/Checkmarx mentions in the bitwarden-security-engineer plugin describe the generic Bitwarden security toolchain present in every repository and are not a dependency on this repo's deleted file. As a CI-infrastructure-only change with no plugin code, docs, or config touched, no plugin version bump or changelog entry is required.

Code Review Details

No findings. The deletion is clean and well-scoped.