removeEdge abstract method
- Edge<
E> edge 
override
    Removes an edge from the graph.
If the edge was removed, the graph must:
- Remove the edge from the graph so that containsEdge returns 
false; - Remove 
targetas a successor ofsourcein successors; - If 
sourcehas no more successors, returnfalsefor containsRoot; 
In addition, the graph may, depending on the implementation:
- Remove 
sourceandtargetfrom the graph if they have no more edges; - Remove the inverse edge, i.e. for undirected graphs.
 
Implementation
@override
bool removeEdge(Edge<E> edge);