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