There is no way to remove a dead field from an element of a List of Model Type

(Bug, Investigate , Priority: High, Test Status: No automated tests yet , Reported By Justin du Coeur, )
Editor, Model Types
Summary: This is rare, but can come up -- it's a design bug.
That is, create Model Type MT, with several fields. Now, on some Thing, create a List of MTs. Add some elements to it, filling in those fields. Now remove one of the MT fields. The elements still have that field, because it's been set locally. Moreover, there is no mechanism in the Editor to remove that field, because there is no Advanced Editor for a single record.
More generally, this bug is that there is no way in the Instance Editor to remove the local value of a field once it is set. That needs to be fixed, somehow.