OSV importer not present

https://github.com/aboutcode-org/vulnerablecode/blob/main/vulnerabilities/importers/__init__.py only states github_osv, OSV itself as a data source is not present.

https://github.com/aboutcode-org/vulnerablecode/blob/main/vulnerabilities/importers/osv.py does not inherit ``Importer`` at all.

Looks like a regression, as there used to be OSV importer earlier.