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 flag was enabled by default in #12608 (closed).

The issue tracking this is #13527 (closed), and is part of the larger feature #10077 (closed).

This MR appears large, but the bulk of it is mostly just deleted (obsolete) snapshots. It's probably best viewed by disabling Show whitespace changes.

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