AUTO_COMPOSED_PROP_IDX unused?

[Eli Zaretskii]

In dispextern.h we have:

    enum prop_idx
    {
      AUTO_COMPOSED_PROP_IDX,
      FONTIFIED_PROP_IDX,
      FACE_PROP_IDX,
      INVISIBLE_PROP_IDX,
      DISPLAY_PROP_IDX,
      COMPOSITION_PROP_IDX,

      /* Not a property.  Used to indicate changes in overlays.  */
      OVERLAY_PROP_IDX,

      /* Sentinel.  */
      LAST_PROP_IDX
    };

However, no one seems to use AUTO_COMPOSED_PROP_IDX.  It looks like a
left-over from the previous implementation of automatic compositions,
which used text properties.

OK to delete AUTO_COMPOSED_PROP_IDX from the enumeration?