scheduler: Clean up dead code about priority/ratio

Following swh/infra/sysadm-environment#3084 (closed), the priority code is no longer relevant.

Priority tasks are still used, they are rerouted by the runner to dedicated queues. The number of tasks with a given ratio per priority is no longer used, that's the part that needs clean up.

Clean it up from the base code.

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Migrated from T3271 (view on Phabricator)