chore: Drop SEC_REGISTRY_IMAGE

What does this MR do?

In the process of updating SEC_REGISTRY_IMAGE to secrets instead of secret-detection, we drop this var instead. The name change adds needless complexity to the transition for the SECURE_ANALYZER_PREFIX and should be avoided. Instead we plan on setting this via CI variables

For this context see https://gitlab.com/gitlab-org/gitlab/-/issues/334325#note_837950172

Note I've already updated SEC_REGISTRY_PASSWORD and SEC_REGISTRY_USER within CI variables so with this MR we should be able to cut a new build tag and push to the relevant registry

What are the relevant issue numbers?

Relates to https://gitlab.com/gitlab-org/gitlab/-/issues/334325

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