Remove without_archived_projects and associated FF

What does this MR do and why?

1. Removes the without_archived_projects since it is no longer being used
2. Removes the vulnerability_scope_disable_cross_join flag since it has been rolled out completely on production.

References

Please include cross links to any resources that are relevant to this MR. This will give reviewers and future readers helpful context to give an efficient review of the changes introduced.

#515252 (closed)

#504568 (closed)

MR acceptance checklist

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Related to #515252 (closed)