phabricator lister: Clean up incremental lister

The incremental lister is not really ready 1. It's not really necessary either (as the phabricator instances out there are not that big [2])

so clean up:

• scheduler db incremental task [3]

• swh-site's scheduler configuration

• swh-lister's code (if it's not used/tested, it's dead code so might as well remove it, less churn in understanding for new people, less maintenance and all that)

• [1] swh/meta$435

• [2] #1801 (closed)

• [3]

14:16:50 softwareheritage-scheduler@belvedere:5432=> delete from task_run where task in (select id from task where type = 'list-phabricator-incremental');
DELETE 5
Time: 12.546 ms
14:17:01 softwareheritage-scheduler@belvedere:5432=> delete from task where type = 'list-phabricator-incremental';
DELETE 5
Time: 9.527 ms
14:17:14 softwareheritage-scheduler@belvedere:5432=> delete from task_type where type='list-phabricator-incremental';
DELETE 1

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Migrated from T1824 (view on Phabricator)