The second viola staff in my example has a unwanted treble clef that’s
impossible to select in either Write mode or Engrave mode
impossible to delete.
An attempt at a work-around (copying the 2nd divisi part onto a 3rd divisi part then deleting the 2nd part) failed when that just merely deleted the 3rd part.
For some reason your entire second staff is a treble clef staff instead of an alto clef. Add a Divisi change in Bar one, then you can delete the treble clef at the beginning. After that you can delete that divisi change again.