I was able to delete it. However I notice that if I delete it only the tables get processes, I notice that sequeces, or packages still have the schema. object name even when the schema has been dropped.
Using 3.1.1 import from 11g, one table and one dimension.
Merge into a fresh relational model.
Open the browser to show both objects and note the schema name is included.
In the browser / Relational Models / Relational_1 / Schemas: delete the schema
Observe that for the Physical Models / Tables that the schema name has disappeared.
Observe that for the Physical Models / Dimensions that the schema name is still present -- inconsistent result w.r.t. to the table object.
Rather than hand fixing it I proceeded to delete the Physical Models / User
then closed and reopened the physical models and -- voila! -- the schema name has disappeared from the dimension
Schema-less models are all that I currently work with and just maybe deleting both the user and the schema, closing and re-opening is a workaround for en masse schema elimination? We'll see.