[Cleanup] Use computeEnergy()

dune/tectonic/ellipticenergy.hh

@@ -23,10 +23,7 @@ template <size_t dim> class EllipticEnergy {
       : A(A), b(b), phi(phi), ignore(ignore) {}
 
   double operator()(SmallVector const &v) const {
-    SmallVector y(0);
-    Arithmetic::addProduct(y, 0.5, A, v);
-    y -= b;
-    return y * v + (*phi)(v);
+    return computeEnergy(A, v, b) + (*phi)(v);
   }
 
   SmallMatrix const &A;