Node get_position_in_parent() and get_index() do the same

[KoBeWi]

I used get_index() a lot, but recently while randomly looking at documentation, I noticed the get_position_in_parent(). It has different description (!), but does the same. I looked at implementation and they literally are the same functions under different names and different descriptions, so it seems like unintended alias.

[eon-s]

Related #16863

[akien-mga]

Both date back to the original open source commit, one should indeed be removed. Moving this to an entry in #16863.

[girng]

index implies it's a child of a node that has a parent