remove private `to_real` member function

It reimplements `std::real` and is not used anywhere.

dune/istl/solvers.hh

@@ -944,19 +944,6 @@ namespace Dune {
 
   private:
 
-    // helper function to extract the real value of a real or complex number
-    inline
-    real_type to_real(const real_type & v)
-    {
-      return v;
-    }
-
-    inline
-    real_type to_real(const std::complex<real_type> & v)
-    {
-      return v.real();
-    }
-
     void generateGivensRotation(field_type &dx, field_type &dy, real_type &cs, field_type &sn)
     {
       using std::sqrt;
@@ -1265,19 +1252,6 @@ namespace Dune {
       return conj(t);
     }
 
-    // helper function to extract the real value of a real or complex number
-    inline
-    real_type to_real(const real_type & v)
-    {
-      return v;
-    }
-
-    inline
-    real_type to_real(const std::complex<real_type> & v)
-    {
-      return v.real();
-    }
-
     void
     generatePlaneRotation(field_type &dx, field_type &dy, real_type &cs, field_type &sn)
     {