ofagent: Remove obsolete bridge_mappings (agent 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For ofagent, it has been superseded by physical_interface_mappings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: I68c7e9991a3dc14ccad709ed1a438c7c45420dd6
Merge "ofagent: Remove obsolete bridge_mappings (agent side)"