Cleanup leftover dependencies to official clients

Some scenario test classes still references to official clients
after the migration. Cleaning them up.

Neutron check_preconditions validates configuration via discovery.
The general approach in tempest is to rely on configuration rather
than discovery of available services and features.
This code uses a neutron client exception, so rather then migrating
it I'm removing it as it's wrong anyways.

Change-Id: I6994a0109c9ec26b290fd4dce9c524dc4a060a54
Partially-implements: bp/tempest-client-scenarios
Merge "Cleanup leftover dependencies to official clients"