model: hack dumpGraph() bug by "leaking" promises

model: hack dumpGraph() bug by "leaking" promises

It is possible to end up in an inconsistent state, where a "resolved"
promise may still be referenced if CycleGraph::resolvePromise() failed,
so don't delete 'promise'. Technically, this leaks memory within an
execution, but because it is allocated on the snapshotting heap, this
leak goes away after an execution ends. So, rather than spending more
work on "fixing" the inconsistency, just don't delete the promise yet.

This inconsistency only matters when dumping the mo_graph to file, since
it traverses all the ModelAction/Promise edges, including those that are
semi-dangling mid-merge.