Stop using portbindings_db in BSN ML2 driver

Avoids using the portbindings_db in the Big Switch
ML2 driver since ML2 has deprecated that database for
its own version that tracks the same information.

Also eliminates unnecessary 'binding_host' field since
it is now always the same as as the 'portbinding:host_id'
field.

Closes-Bug: #1300628
Change-Id: I17d47cb446cdd2e989c3e0d01b797a81309faaa7
Merge "Stop using portbindings_db in BSN ML2 driver"