TO unable to unassign ORG server from delivery services if it's the last assigned

[rawlinp]

I'm submitting a ...

• bug report

Traffic Control components affected ...

• Traffic Ops

Current behavior:

As of #5328, TO prohibits unassigning all servers from an active delivery service, including ORG servers.

Expected behavior:

This validation should be limited to server types that begin with EDGE, since the validation should be designed to prevent the accidental removal of all EDGE-type servers from an active delivery service.

Minimal reproduction of the problem with instructions:

Create an active delivery service and assign an ORG server to it. Try to remove the ORG server from the active delivery service.

Anything else:

Once fixed, this TODO should be completed so that the test properly cleans up after itself.

[srijeet0406]

Just mentioning this here to avoid any confusion:
Here is how I tested and verified this --

Create a new active DS in TP
Try to assign a server to this DS that is not in the ONLINE/ REPORTED state -> this should fail
Now, try to assign an EDGE that is in ONLINE/ REPORTED state -> this should pass
Try deleting this EDGE server from TP and also from the backend API call -> both these actions should be prevented

Now create a new active DS in TP
Assign an ONLINE/ REPORTED ORG server to this DS from TP -> this should pass
Now, try to unassign this server from TP -> this should be prevented, because TP uses the replace=true mode, and you cannot unassign(or replace) all servers from an active DS
Try to delete this server from the backend call -> this should pass, since this is an ORG server

Note: to make a backend call to the DELETE api, use this endpoint:
DELETE https:///api/3.0/deliveryserviceserver//