xAODTriggerAthenaPool, xAODTrigger: Remove unneeded custom pool converters.
xAOD objects no longer need custom pool converters. Remove those that are no longer needed. Change xAOD converters with versioning to use the templated pool converter, and make the TP class interfaces const.
Make the pool converter a friend of TriggerMenuAuxContainer, to allow removing a #define private downstream.