ThinningUtils: Fix neutral PFO thinning.
JetETMissLCNeutralFlowElements is a shallow copy of JetETMissNeutralFlowElements. So if we thin the latter, we also need to thin the former in the same way.
JetETMissLCNeutralFlowElements is a shallow copy of JetETMissNeutralFlowElements. So if we thin the latter, we also need to thin the former in the same way.