removeRelation method
Implementation
void removeRelation(ModelRelation rel) {
final foundRel = findSameRelation(rel);
if (foundRel == null) {
throw StateError(
"cannot remove relation '${rel.name}' with id ${rel.id}: not found");
}
_relations.remove(foundRel);
model.retiredRelationUids.add(rel.id.uid);
}