ExternalizedSingletonInverseVariableSupply doesn't need to keep a Map of Sets after PLANNER-252

Thanks to PLANNER-252, the ExternalizedSingletonInverseVariableSupply doesn't need to account for multiple inverses for 1 value,
because the first inverse.afterVariableChanged() is called after the last variable.before/afterVariableChanged() which calls all inverse.beforeVariableChanged().