RecoVertex/MultiVertexFit: clean -Wunused-function warnings

Signed-off-by: David Abdurachmanov <David.Abdurachmanov@cern.ch>
Merge pull request #2556 from davidlt/clean-unused-function-RecoVertex-MultiVertexFit

GCC 4.9.0 -- RecoVertex/MultiVertexFit: clean -Wunused-function warnings