Remove support for AGMG

From opm/opm-core#308:
> OPM ... should [not] be in the business of facilitating closed systems
> ...
>  the incentive for using AGMG, ... is gone
> ...
> One possibility would be to ... just simply to remove it entirely

If it is deemed unfortunate to use it because the results would not be freely reproducible, and there is a free and just as good alternative, then having it in the code just means more dead-weight to lug along which we don't really want anyone to use anyway.

(If you think you may want to resurrect it later (for instance if the license to AGMG changes back to open source), you may want to consider merging opm/opm-core#308 first to have a commit that links, and then I'll rebase this one on top of that)