This was a regression from 4b60c6ca27. The original plan for introducing
new fields was to have them save to the sync cache even if they weren't
supported by the current Zotero version and process them on upgrade, and
so I changed `DataObjectUtilities.patch()` to omit fields that didn't
exist locally when patching the sync cache JSON so they wouldn't be
wiped on the server. That caused this bug where locally deleted fields
were restored on every sync. It's also no longer necessary now that
we decided to just reject unknown fields from saving, so we can just
revert to the previous behavior of blanking out locally missing fields
(with the tweak that fields that are already false or empty in the base
version can be omitted).
52fd91950d