remove wrong scaling of the energy

[[Imported from SVN: r11301]]

dune/elasticity/materials/neohookeanmaterial.hh

@@ -131,7 +131,7 @@ public:
             }
         }
             
-        return 0.5*energy;
+        return energy;
     }
    
     //! Return the local assembler of the first derivative of the strain energy