Silenced a few warnings

Signed-off-by: Ian Bell <ian.h.bell@gmail.com>

src/SpeedTest.cpp

@@ -11,7 +11,6 @@ namespace CoolProp{
 void compare_REFPROP_and_CoolProp(std::string fluid, int inputs, double val1, double val2, std::size_t N, double d1, double d2)
 {
     time_t t1,t2;
-    double dx = 1/((double)N);
 
     shared_ptr<AbstractState> State(AbstractState::factory("HEOS", fluid));
     t1 = clock();