Remove dashboard settings component and use project manager instead

What does this MR do?

For the instance-level vulnerability report, there is a Settings page to add/remove projects to the report:

This MR removes an unnecessary component and moves the logic it was handling into a more appropriate component (more details in comments).

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because this is a refactor.
• [-] 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
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done