Do not add default OPTE route to internet gateway

[bnaecker]

As part of supporting oxidecomputer/opte#167, the OPTE external IP workaround, we added a default route to OPTE's router layer that sent all traffic to the internet gateway. Specifically we pointed 0.0.0.0/0 to the gateway. That is actually a useful default rule that appears in the VPC System Router, but we don't want to set it this way. It should come as part of a complete set of routes we need to provide from Nexus to OPTE.