Turn off MET significance calculation in CaloMETs

The previous pull request: https://github.com/cms-sw/cmssw/pull/4838
might cause potential problem in future. As suggested by Tai, we
can just turn off MET significance calculation in CaloMETs.
Merge pull request #4867 from zhenbinwu/JetMET_TP_SLHC16

Turn off MET significance calculation in CaloMETs for SLHC