storage.writer: Fix journal writer sanitizer function

According to the value_sanitizer docstring, this takes 2 parameters, first is the object type, the second is the actual dict value to sanitize. As a somewhat default identity function, this discards the object type and returns directly the dict value unchanged.

• [1] https://forge.softwareheritage.org/source/swh-journal/browse/master/swh/journal/writer/kafka.py$97-100

Test Plan

tox

vagrant is ok with patched indexer storage, running the actual swh-indexer-journal-client up to writing to the new swh.journal.indexed.* topics.

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Migrated from D4624 (view on Phabricator)