MagFieldServices: load field map during start for online/HLT running

Load the magnetic field map during `start()` for online/HLT running.
This was already configured correctly in the new-style config fragment
but not in the old version.

This fixes the problem of `AtlasFieldMapCondAlg` running once per slot in
the HLT.

MagneticField/MagFieldServices/python/MagFieldServicesSetup.py

-# Copyright (C) 2002-2020 CERN for the benefit of the ATLAS collaboration
+# Copyright (C) 2002-2021 CERN for the benefit of the ATLAS collaboration
 
 # JobOption fragment to set up the magnetic field services and algorithms 
 # Valerio Ippolito - Harvard University
@@ -26,10 +26,9 @@ def AtlasFieldCacheCondAlg(name="AtlasFieldCacheCondAlg",**kwargs):
 
 def AtlasFieldMapCondAlg(name="AtlasFieldMapCondAlg",**kwargs):
   if athenaCommonFlags.isOnline():
-    # For old tests, must update field from defautl jobOpt
+    # online has the map loaded at start and does not use DCS
     kwargs.setdefault( "UseMapsFromCOOL", False )
-    # online has the map loaded at start (for the future - uncomment if needed)
-    # kwargs.setdefault( "LoadMapOnStart", True )
+    kwargs.setdefault( "LoadMapOnStart", True )
 
   return CfgMgr.MagField__AtlasFieldMapCondAlg(name,**kwargs)