Fix warning and remove unused setReference(strm) for generator-vel-ref.

src/Mathematics/FootConstraintsAsLinearSystemForVelRef.cpp

src/ZMPRefTrajectoryGeneration/generator-vel-ref.cpp

src/ZMPRefTrajectoryGeneration/generator-vel-ref.hh

src/ZMPRefTrajectoryGeneration/qp-problem.cpp

src/privatepgtypes.cpp