swh-loader-tar generates dangling releases

The releases generated by swh-loader-tar aren't referenced by any occurrences: we generate occurrences pointing at revision objects, but not at releases.

Actions:

• stop generating dangling releases and keep the occurrences pointing at revisions (source code adaptation)
• Remove dangling gnu releases (generated by the tarball loader when ingesting gnu tarballs)

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Migrated from T683 (view on Phabricator)