Delete outdated sub-ecosystem data in`osv-vulnerabilities` bucket

The `osv-vulnerabilities` bucket currently stores all vulnerabilities exported by the exporter, including historical entries (no auto deletion). We recently modified the exporter to only export vulnerabilities for the main ecosystem (e.g. Debian, Ubuntu) instead of individual sub-ecosystems (e.g. Debian:11, Debian:12). Because of this change, a large number of legacy sub-ecosystem directories remain there.

To maintain a cleaner bucket, we should remove these outdated directories and files.

If we also want to remove nonexistent vulnerability records from each ecosystem, this could resolve issues like https://github.com/google/osv.dev/issues/2902