RecoVertex/MultiVertexFit: clean -Wunused-function warnings

Cleans GCC 4.9.0 -Wunused-function warnings.

Also in `RecoVertex/MultiVertexFit/src/MultiVertexReconstructor.cc` there is a method `inline void remove (...)`, which uses `std::cout`. Maybe that should be converted?

Signed-off-by: David Abdurachmanov <David.Abdurachmanov@cern.ch>