Remove support for AGMG from the build system

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.

cmake/Scripts/configure

@@ -39,10 +39,6 @@ Optional Packages:
   --with-opm=PATH         specify parent of all OPM modules not specified
   --with-opm-MODULE=PATH  use given OPM module from a specified location
   --with-superlu=PATH     user defined path to SuperLU library
-  --with-agmg=PATH        Include DOUBLE PRECISION version Notay's of AGMG
-                          Algebraic Multigrid solver from specified source
-                          location. Note: this option requires a complete,
-                          working Fortran 90 environment.
   --with-umfpack=PATH     use UMFPACK/SuiteSparse from a specified location
   --with-ert=PATH         Use ERT libraries
   --with-tinyxml=PATH     use TinyXML library from a specified location
@@ -187,7 +183,6 @@ for OPT in "$@"; do
             mpi-prefix)
               rootvar="_MPI_PREFIX_PATH"
               ;;
-            agmg        |\
             ert         |\
             boost       |\
             superlu     |\
@@ -280,7 +275,6 @@ for OPT in "$@"; do
               use_ninja=""
               pkgname=""
               ;;
-            agmg |\
             ert  |\
             superlu)
               pkgname="${pkgname^^}"