code: public Object remove(Object vertex) {
          VertexMetadata vm = (VertexMetadata) vertices.get(vertex);
          if (vm == null) {
              return null;
          }
          vertices.remove(vertex);
          vertexModCount++;
          int index = vm.index;
          for (int row = 0; row < capacity; row++) {
              edges[row][index] = null;
              edges[index][row] = null;
              edgeModCount++;
          }
          edgesArr = null;
          freeList.add(index);
          return vertex;
      }
docs: Remove a vertex and all edges between the vertex and other vertices.
  
  @param vertex
  a vertex.
  @return the value of the vertex, if it existed.