code-cleaning in RecoMET

Eventually, we will delete "significance" from "CorrMETData" as we will make "CorrMETData" a 2D vector on the px-py plane as it was discussed in cms-metdevelopers@cern.ch a long time ago.
https://github.com/cms-sw/cmssw/blob/CMSSW_7_1_0_pre6/DataFormats/METReco/interface/CorrMETData.h#L21

Also we will eventually delete "corr" from "MET" as this is not used by most MET corrections (I think that this is used only in the correction of CaloMET for muons. This is not used by Type-I/II/0 corrections)
https://github.com/cms-sw/cmssw/blob/CMSSW_7_1_0_pre6/DataFormats/METReco/interface/MET.h#L76

This PR request will prepare for these deletion.