Makes using wrong constructor a compile-time error

Petsc only supports initialisation through the ParameterGroup constructor.
Calling the default, non-arg constructor is a static error, and not
implementing it makes using it break compiles.

opm/core/linalg/LinearSolverPetsc.cpp

@@ -219,11 +219,6 @@ namespace{
 
 } // anonymous namespace.
 
-    LinearSolverPetsc::LinearSolverPetsc()
-    {
-        OPM_THROW(std::runtime_error, "Pestc just can be called through paramers.\n");
-    }
-
     LinearSolverPetsc::LinearSolverPetsc(const parameter::ParameterGroup& param)
         : ksp_type_("gmres")
         , pc_type_("sor")

opm/core/linalg/LinearSolverPetsc.hpp

@@ -33,14 +33,10 @@ namespace Opm
     {
     public:
         /// Default constructor.
-        /// All parameters controlling the solver are defaulted:
-        ///   ksp_type                      gmres
-        ///   pc_type                       jacobi
-        ///   ksp_view                      0
-        ///   ksp_rtol                      1e-5
-        ///   ksp_atol                      1e-50
-        ///   ksp_dtol                      1e5
-        ///   ksp_max_it                    1e5
+        /// Declared, but not implemented. Petsc can only be created through
+        /// the ParameterGroup constructor, everything else is an error. This way
+        /// the error is caught compile time and not rune time, which is nice as
+        /// it is a static error.
         LinearSolverPetsc();
 
         /// Construct from parameters