Remove explicit support for OneConvergence plugin

This is being removed from the Neutron tree, so there is
no need to keep it here anymore.

Change-Id: Ice869bc445cb9dab6f227c30d38fb9b7ba04442b
Depends-on: I949a51873ee5af654b577952d423dd29a6ced8e7

Author: armando-migliaccio