ISF_Fatras: Only create PhysicsValidationTool when ValidationMode is activated

I noticed that the PhysicsValidationTool was being created in every ATLFAST3F_G4MS job even if flags.Sim.ISF.Validation was set to False. This merge request ensures that the ToolHandle is disabled when not required. (Prevents some warnings and should save a small amount of memory.)

Also demoted some INFO messages to DEBUG.