Eliminate need for static port mappings for switch ports

[internet-diglett]

omicron/nexus/src/app/rack.rs

Line 298 in 973bff1

ExternalPortDiscovery::Static(port_mappings) => {

Currently we are passing in a static port mapping configuration in some of our test cases. Now that we can
dynamically discover this from a dendrite instance, and every test context is running a dendrite instance,
it would be nice to eliminate the need for static mappings.