ofagent: Remove obsolete bridge_mappings (plugin side)

For ofagent, it has been superseded by physical_interface_mappings
and was planned to be removed in Kilo.

Related: blueprint ofagent-physical-interface-mappings
Change-Id: I1f7cbcde429a03e7e918dc224d8cbd8c6aa47049
Merge "ofagent: Remove obsolete bridge_mappings (plugin side)"