Changes non-const statics in PFCluster to avoid static analyzer warnings

The static analyzer flagged several non-const statics in this class as potential threaded problems. We removed instanceCounter_ since it was never used and moved the dummyVtx function static used as a return value to be a const
class static. The statics associated with setDepthCorParameters and getDepthCorParameters were left as is but they were reported to the appropriate L2s.