drop using statement for well_index_

rather qualify member variable with this->

opm/simulators/wells/StandardWell.hpp

@@ -262,7 +262,6 @@ namespace Opm
 
     protected:
         // protected member variables from the Base class
-        using Base::well_index_;
         using Base::index_of_well_;
         using Base::num_components_;
         using Base::connectionRates_;