Raise NotImplementedError in NeutronV2 API

Remove unused code in nova.network.neutronv2 that returns an empty
list. Instead of just returning an empty list, let's be more explicit
and raise a NotImplementedError. This will be more descriptive if
someone is trying to use this part of the NeutronAPI.

Commit 45e92d4703 contains the changes to use the db layer instead of
the network rpcapi.

Change-Id: I2a9ada8186fd4165c7fceb15965055b5793e6e2c
Closes-Bug: 1258380
Merge "Raise NotImplementedError in NeutronV2 API"