removeEdge abstract method

Removes an edge from source to target with a weight to the graph.

Returns the weight of the edge, or null if the edge was not present.

If the edge was removed, the graph must:

• Remove the edge from the graph so that containsEdge returns false;
• Remove target as a successor of source in successors;
• If source has no more successors, return false for containsRoot;

In addition, the graph may, depending on the implementation:

• Remove source and target from the graph if they have no more edges;
• Remove the inverse edge, i.e. for undirected graphs.