remove unused cfi file in RecoMET

This PR removes RecoMET/METProducers/python/CaloMETSignif_cfi.py.

The file defines an instance of cms.EDProducer *metsignificance*, which is no longer used or runs.

First, METProducer doesn't exist anymore https://github.com/cms-sw/cmssw/pull/2283.

Second, even when METProducer existed, the METtype *CaloMETSignif*, which appears to have been originally introduced for CMSSW_2_1_0 (https://github.com/cms-sw/cmssw/commit/7831ba34182572985f356422c320928c44feb435#diff-b3e852dcb6b303b860a1e02c15116c9d), was deleted for CMSSW_3_1_0 (https://github.com/cms-sw/cmssw/commit/9766752a10e787a23d154d766735f15461dda084#diff-b3e852dcb6b303b860a1e02c15116c9d).