Remove start.sh script as it's no longer needed

What does this MR do?

This MR removes the container-scanner/start.sh script which was added in !1 (merged). This script is no longer needed now that Remove Container Scanning kubernetes workaround has been completed.

What are the relevant issue numbers?

gitlab-org/gitlab#214413 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated for GitLab EE, if necessary
• Documentation created/updated for this project, if necessary
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Job definition updated, if necessary
  • Auto-DevOps template
  • Job definition example
  • CI Templates
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer