Remove bindings to AGMG from library

AGMG is now under a closed-source license, meaning that results
obtained with this solver is not freely reproducible by others.
Its use is therefore discouraged.

As of version 2.3, the DUNE AMG parts are competitive, so there
is a free and open alternative.

opm/core/linalg/LinearSolverFactory.cpp

@@ -31,11 +31,6 @@
 #include <opm/core/linalg/LinearSolverIstl.hpp>
 #endif
 
-#if HAVE_AGMG
-#include <opm/core/linalg/LinearSolverAGMG.hpp>
-#endif
-
-
 #include <opm/core/utility/parameters/ParameterGroup.hpp>
 #include <opm/core/utility/ErrorMacros.hpp>
 #include <string>
@@ -75,12 +70,6 @@ namespace Opm
 #endif
         }
 
-        else if (ls == "agmg") {
-#if HAVE_AGMG
-            solver_.reset(new LinearSolverAGMG(param));
-#endif
-        }
-
         else {
             THROW("Linear solver " << ls << " is unknown.");
         }