Move getPerfCellPressure to wellInterface

The cell pressure is independent of well model and belongs to the interface
This should move the MSW model one step closer to supporting GasWater cases

opm/simulators/wells/StandardWell.hpp

@@ -263,8 +263,6 @@ namespace Opm
 
 
     protected:
-        Eval getPerfCellPressure(const FluidState& fs) const;
-
         // xw = inv(D)*(rw - C*x)
         void recoverSolutionWell(const BVector& x, BVectorWell& xw) const;