journal: Drop datetime conversion indirection

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] swh-model!278 (closed)

• [3] https://jenkins.softwareheritage.org/job/DSTO/job/tests/1298/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/

Test Plan

tox

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Migrated from D3345 (view on Phabricator)