Merge pull request #186 from Unity-Technologies/fix-nodes-are-still-filled-after-edge-deletion

[FIX] fixes #96 Needed to put the input and output of the edge to Null.

MaterialGraphProject/Assets/UnityShaderEditor/Editor/Drawing/Views/GraphEditorView.cs

                     }
                     edgeView.output.Disconnect(edgeView);
                     edgeView.input.Disconnect(edgeView);
+
+                    edgeView.output = null;
+                    edgeView.input = null;
+
                     m_GraphView.RemoveElement(edgeView);
                 }
             }