Looks like we need a more-specific version of [Graph.removeEdge()](https://github.com/noflo/noflo/blob/db95c95152a6001adcd901dce3a4227259c9d9bb/src/lib/Graph.coffee#L495-L527) for edges with index. (moved from https://github.com/noflo/noflo/issues/254)