Delete old pipelines security report

What does this MR do?

This completely removes the old security reports view and pipeline_report_api feature flag. The view was put behind that flag in !15682 (merged). That feature flag was enabled by default (i.e., to hide the old view in favour of the Security Dashboard) in !15824 (merged).

Related to &1425 (closed).

Screenshots

See !15682 (merged) for screenshots of the old view that's been removed, and what replaced it.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• [-] Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Availability 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