fix typo from 302c1fe542b646a07dfce94172e4a61345768354, refs #17837.
remove completed TODO comment, it's handled by _isParentChildDrop(), refs #7140
Disallow drop onto own parent
unless user is dragging multiple nodes and at least
one of them is *not* being dropped onto its own parent.

In that case, no-op for node(s) being dropped onto their own parent.

Fixes #16692