Remove Legacy L1 monitoring in TauTrigger monitoring

As mention by @tamartin in !77492, the TauTrigger monitoring is producing lots of warning messages related to failing retrieving EmTauRoI from Legacy L1 monitoring part. This is because Legacy is used as fallback solution in case the trigger item is not a Phase-1 trigger. Given that Legacy tau chains have been removed from the menu already in 2024 data taking, this MR is removing the Legacy L1 monitoring from TauTrigger monitoring code

Tagging @slai, @smwang, @mashamim, @jbeaucam and @martindl for tau trigger