dotgen cluster: fix memory leaks by freeing data from removed rank leaders

dotgen cluster: fix memory leaks by freeing data from removed rank leaders

The `build_skeleton` function inserts virtual nodes and edges into the
graph to be used by the clustering algorithms. The `expand_cluster`
function later installs real nodes or sub-clusters and deletes the
virtual nodes and edges by calling the `remove_rank_leaders` function
which removes these from the graph. However, it didn't free the data
allocated for them which caused several memory leaks.