I agree: there is no reason that the Pope cannot remove Burke from the College of Cardinals.  As you say, one is not ordained to the cardinalate.  I think that if there is a finding that Cardinal Burke deliberately misrepresented the pope, or sat passively as someone else misrepresented him, I think that betrayal should have serious consequences.  I'm really not sure what those consequences should be, but the loss of the title, privileges and obligations of the cardinalate could certainly be on the table.