Magnetic field MT migration: remove AtlasFieldSvc (ATLASRECTS-5432)
Remove AtlasFieldSvc and adapt remaining trigger clients.
EDIT: TrigOpMonitor https://gitlab.cern.ch/atlas/athena/-/blob/master/HLT/Trigger/TrigMonitoring/TrigOnlineMonitor/src/TrigOpMonitor.cxx#L118 needs to be migrated but will just give a warning for now.
Edited by Stewart Martin-Haugh