Add new delete mapping method in PortMapping

The current deleteMappingData method of PortMapping corresponds to the deletion of a complete node. It would be more appropriated to rename this method "deletePortMappingNode" for example, and implementing a new deleteMapping method that would delete a given mapping for a given node. Such an operation is required during deletion of OpenRoadmInterfaces, as soon as the mapping contains information with any supported interface that mus be removed.