As Christian notes below, the dev tools come with an app called "Syncrospector." It does what you want.

Open it up, select "Clients" from the pull-down menu in the top left corner. Scroll through the list and select the offending item, and click "Unregister." Select "Schema" from the pull-down, and remove the desired entry(-ies) there, too.

Don't touch anything else, because it's possible/likely you'll end up with missing data. After all, you're tinkering in the same realm that Apple recommends be kept away from "as if it were a swarm of bees."