To do this:
- Open the object definition (e.g., Contact.object).
- Delete the picklistValues elements that are no longer desired.
I had to prune a list of 485 picklist values in a language selection picklist to a more reasonable number.
Regardless of this workaround, it would be great if this feature was more easily accessible through the web UI, as per the following idea: "mass delete picklist values (setup)"
Apparently, all I succeeded in doing was shifting the order of the picklist values. At this point I am also at a loss as to how to mass delete picklist values, much to my chagrin.