fix option for turning off MVA correction (follow up to !71196)

Not sure how this slipped through the last time, but the way the EgammaCalibrationAndSmearingTool turns off the MVA correction is that it doesn't initialize the MVA service. While that is a reasonable approach it only works if the MVA ServiceHandle is not given a default value during construction. This MR should achieve that by removing the default instance name.

I asked our CP algs configuration developers to provide an easier way to set up tests for custom CP algs configurations. Right now it involves quite a bit of boiler-plate code to set up the job. So, while I tested this locally, I'd rather defer putting in a full test setup until it becomes easier to do so.