Small MET validation change - don't make METReference or TrackMET histograms
The METReference and TrackMET histograms have been empty in all MET validation tasks for two years, as AODs don't contain the relevant containers anymore. This occasionally causes confusion as MET validation tasks get flagged as "red" due to these empty histograms.
This change removes the histograms entirely unless the m_doMETRefPlots
flag (introduced in !45261 (merged)) is set to True.
Tagging @snechaev @acervell @mswiatlo @cdelitzs @wbalunas @turra