changed: get rid of OPM_UNUSED macro usage

prefer anonymous parameters and c++17 [[maybe_unused]]

opm/models/flash/flashprimaryvariables.hh

@@ -99,8 +99,8 @@ public:
      */
     template <class FluidState>
     void assignMassConservative(const FluidState& fluidState,
-                                const MaterialLawParams& matParams OPM_UNUSED,
-                                bool isInEquilibrium OPM_UNUSED= false)
+                                const MaterialLawParams&,
+                                bool = false)
     {
         // there is no difference between naive and mass conservative
         // assignment in the flash model. (we only need the total