Disable the sourceforge lister origins

This is a temporary workaround the time we make a first pass on those repositories.

This will allow to list sourceforge without immediately triggering the scheduling of those origins (as we cannot ensure we would respect the limit of 8 tasks in // otherwise).

Related to swh/infra/sysadm-environment#3350 (closed)

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Migrated from D5803 (view on Phabricator)