serializers: Make kafka_to_key implem compatible with stable version
The current debian storage build on stable [1] fails on an unexposed parameter
yet. The current journal build on stable probably because it missed a test on
that method.
This commits removes the need to turn off such parameter. It's not needed for
the kafka objects (no dict with null keys). Which allows to be compatible with
current version onward as well.
[1] https://jenkins.softwareheritage.org/view/Debian%20packages/job/debian/job/packages/job/DSTO/job/gbp-buildpackage/177/console