Removing confusing TripletFields

After additional discussion with rxin, I think having all the possible `TripletField` options is confusing.  This pull request reduces the triplet fields to:

```java
  /**
   * None of the triplet fields are exposed.
   */
  public static final TripletFields None = new TripletFields(false, false, false);

  /**
   * Expose only the edge field and not the source or destination field.
   */
  public static final TripletFields EdgeOnly = new TripletFields(false, false, true);

  /**
   * Expose the source and edge fields but not the destination field. (Same as Src)
   */
  public static final TripletFields Src = new TripletFields(true, false, true);

  /**
   * Expose the destination and edge fields but not the source field. (Same as Dst)
   */
  public static final TripletFields Dst = new TripletFields(false, true, true);

  /**
   * Expose all the fields (source, edge, and destination).
   */
  public static final TripletFields All = new TripletFields(true, true, true);
```

Author: Joseph E. Gonzalez <joseph.e.gonzalez@gmail.com>

Closes #3472 from jegonzal/SimplifyTripletFields and squashes the following commits:

91796b5 [Joseph E. Gonzalez] removing confusing triplet fields