Remove dependency_list_vulnerabilities feature flag

What does this MR do?

This removes all code/template/spec branches associated with the missing/disabled case of the dependency_list_vulnerabilities feature flag.

The issue tracking this is #12608 (closed), and it part of the larger feature #10077 (closed).

Note: WIP, since it's blocked by #13190 (closed).

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• [-] Style guides
• [-] Database guides
• Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers