swh.lister.gnu: Change origin type to tar

Change origin type from 'gnu' to 'tar'

Rationale, there is no need for differentiating origin-type 'gnu' from other origin-type 'tar'. It's actually already mentioned in the origin's entry (through its url).

Furthermore, from an historic point of view, we shall try to not diverge too much from original gnu loading. Even though, those were typed ftp at the time... (We were young and foolish back then. Personally, I still am but i like to believe i'm a little less foolish now ;)

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Migrated from D1612 (view on Phabricator)