ofagent: Desupport ancillary bridges

Remove code to report port status for ancillary bridges because
it relies on ovsdb.  While we could adapt the code to use OFP in
a similar way we did for the integration bridge, it doesn't worth
the effort because the almost same functionality can be achieved
using provider network.  Update README to suggest the use of
provider network.

Partially-Implements: blueprint ofagent-port-monitor
Change-Id: Ia1329e36755b3e9a479a115dbd08aa3f4d6b55df