Django 2: Remove conflicting exclude for CommCareCaseSQLSerializer.case_json
Django 2: Remove conflicting exclude for CommCareCaseSQLSerializer.case_json

It looks like this exclude was being ignored in djangorestframework 3.5
but that some later versions check to make sure it's either in exclude
or set as a property.

Digging back to how we got here, it looks like it was two PRs @czue and
@snopoke worked on separately but at the same time and in communication
with each other: @czue's https://github.com/dimagi/commcare-hq/pull/10560/files#diff-386b068280d3d01753890cf76a65da50R46
and @snopoke's https://github.com/dimagi/commcare-hq/pull/10581/files#diff-386b068280d3d01753890cf76a65da50R50.
It looks like @czue's change to add the case_json property with an
explicit type won out over @snopoke's to exclude that property,
but both ended up in the code. I'm guessing no one noticed because even
though both ended up in the code, it happened to have the intended
result anyway.

dimagi/commcare-hq,dimagi/commcare-hq,dimagi/commcare-hq,dimagi/commcare-hq,dimagi/commcare-hq