[Cleanup] Do not recompute x.two_norm()

dune/tectonic/localfriction.hh

@@ -36,7 +36,7 @@ template <int dimension> class LocalFriction {
     if (xnorm < 1e-14 && xnorm >= smp)
       return std::numeric_limits<double>::infinity();
 
-    return func->regularity(x.two_norm());
+    return func->regularity(xnorm);
   }
 
   // directional subdifferential: at u on the line u + t*v