This reverts commit c873e8c92da389bc1d6bc4ed2a5241faddfa7630.
since OPM/opm-simulators#1287 has been merged there are no "in tree"
upstreams which use that mechanism anymore.
this now works with the unmodified master version of flow from
opm-simulators. we take the liberty to emit a deprecation warning,
though. this complicates things quite a bit.