World Partition: warnings on editor only actors when running the editor in -server/-game mode

The warning for editor-only actors is unnecessary in these modes where they don't exist.

• Create a level that has Editor Utility Blueprint-derived actors.
• Run the editor in server/game mode (with -server or -game command lines).
• Observe warnings:
  • 024.04.08-17.03.03:022][ 0]LogWorldPartition: Can't find class descriptor '/Game/Main/EditorTools/EUBP_Tool.EUBP_Tool_C' for loading '/Game/Main/Maps/TestMap.TestMap:PersistentLevel.EUBP_Tool_C_147', using '/Script/Engine.Actor'
  • [2024.04.08-17.03.03:023][ 0]LogWorldPartition: Warning: Invalid class for actor guid 3XXXXXXXXXXXXXXXX ('EUBP_Tool_C_147') from package '/Game/ExternalActors/Main/Maps/TestMap/0/1A/XXXXXXXXXXXXXXXXXX'