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.
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.
Merge pull request #314 from rolk/314_agmg

Remove support for AGMG