Search node address in port's DiscoveredBindings instead RealizedBindings

[heypnus]

In IPI + NSX-T impactor, the node LSP's addresses are in discovered_bindings instead of realized_bindings.

This patch also will remove some stale functions. In previous NSX-T, the addresses locate in both realized_bindings and discovered_bindings, so we don't need to worry about the upgrade case.