Remove obsolete constructors

dune/tectonic/nicefunction.hh

@@ -26,8 +26,6 @@ class NiceFunction : public VirtualFunction<double, double> {
 
 class RuinaFunction : public NiceFunction {
 public:
-  RuinaFunction() {}
-
   RuinaFunction(double coefficient, double a, double mu, double eta,
                 double normalStress)
       : coefficient(coefficient),
@@ -112,8 +110,6 @@ class RuinaFunction : public NiceFunction {
 
 class LinearFunction : public NiceFunction {
 public:
-  LinearFunction() {}
-
   LinearFunction(double a) : coefficient(a) {}
 
   void virtual evaluate(double const &x, double &y) const {