[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 removes the deprecated scan.yml GitHub Actions workflow (Checkmarx SAST and Sonar quality jobs) under VULN-649. The equivalent Checkmarx and Sonar jobs are preserved in scan-ci.yml, which runs on push to main and via manual dispatch. A repository-wide search confirms no remaining references to scan.yml, so this is a clean removal with no orphaned configuration.

Code Review Details

No blocking findings.

Note: scanning cadence shifts from per-PR (old scan.yml used pull_request/pull_request_target) to post-merge on main (scan-ci.yml). This appears intentional per VULN-649 and is an AppSec process decision rather than a code concern.