xAOD skimming cleanup

There are currently a few inconveniences related to the xAOD skimming which mostly just show up when re-skimming already skimmed samples, e.g.,

• the flag indicating an event should be kept is not removed before copying the event to the output and, hence, the flag is persistified
• re-skimming an already skimmed sample tends to unnecessarily increase the file length by prepending things like CAFxAOD_... again. This should be smartly detected and avoided