When nullifying [1] the origin-visit fields, i came across that code which
is no longer necessary.
That is even the source of trouble in diffs and master. [3]
It's most possibly introduced by [2]
[1] Depends on D3344
[2] D3336
[3] https://jenkins.softwareheritage.org/job/DSTO/job/tests/1299/console
https://jenkins.softwareheritage.org/job/DSTO/job/tests-on-diff/347/testReport/junit/.tox.py3.lib.python3.7.site-packages.swh.storage.tests/test_backfill/test_backfiller/