Nicira plugin: Remove chained logical switches support

The nicira plugin has a mechanism for chaining multiple logical switches on the same neutron network.
This mechanism only applies for networks with a provider mapping of type "flat" or "vlan", and was introduced when the NVP backend had scale limitations for this kind of networks.

These limitations have been lifted from the backend over a year ago, and keeping this mechanism causes severe inefficiencies because of rather expensive queries performed by the Neutron plugin.

This bug fix will remove this support and provide a mechanism to convert existing deployments which chained networks.