DERP Relay URL shouldn't be required if DERP server is disabled #12493
Description
Technically it's not required, but if you don't specify one (because you are using external DERPs) and run HA you get log spam:
[erro] coderd.derpmesh: failed to add address address=/derp ...
error= create derp client:
github.com/coder/coder/v2/enterprise/derpmesh.(*Mesh).addAddress
/home/runner/actions-runner/_work/coder/coder/enterprise/derpmesh/derpmesh.go:105
- derphttp.NewClient: invalid URL scheme ""
If we see that the internal DERP server is disabled, we should not create a derpmesh or derp.Server, and we should not deal with DERP in replicasync.