Removes the unknown overview from the group security dashboard

What does this MR do?

Removes the "unknown" category from the group security dashboard overviews.

Before	After

What are the relevant issue numbers?

#6709 (closed) #8200 (closed)

Does this MR meet the acceptance criteria?

• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?