Turn off MET significance calculation in CaloMETs for SLHC

The previous pull request: https://github.com/cms-sw/cmssw/pull/4838
might cause potential problems in future. As suggested by Tai Sakuma, 
we can just turn off MET significance calculation in CaloMETs.

This pull request reverts the previous changes and turns off MET 
significance calculation in RecoMET/METProducers/python/CaloMET_cfi.py.
The MET significance will be set zero in these MET objects.