Don't unset src, dst in edge itself; just remove from the model

[majensen]

Consistent with handling properties in rm_node (props are removed from model, but not from the node itself), want to leave src and dst nodes connect to an edge when de-registering the edge from the model.

bento-meta/perl/lib/Bento/Meta/Model.pm

Line 403 in 172e3ed

$edge->set_src(undef);