onnxruntime

Fix tree implementation when left, right node have lower index (#14839)

### Description
Previous implementation did not support left or right node of a node to
have an index lower than the node itself. This condition would forbid
the tree to enter an infinite loop. Lightgbm does not follow that rule.
The changes do not change the algorithm but remove the test enforcing
that condition.



### Motivation and Context
It fixes a regression introduced by #14670.