Actually just did a quick test in a NSX-T 3.0 environment, "remove the edge cluster from the T0 and save...There is no error..." might be a bug...If I switch to Manager role, the newly created T0 is still associated with the edge cluster even there is no Edge cluster shown in Policy role...
If you need to host stateful services such as NAT, load balancer, or firewall you need to select Edge Cluster. That being said a service router will always be created when we select an edge cluster even if you don't use/configure a stateful service feature.