[FIX] stock: traceability report: direct children displayed

Configure a route to be in 3 steps
make a PO, validate all three pickings
Go on the final picking, the one that has the destination to the Stock
Click on the smart button to access the traceability report
unfold as many lines as you can

Before this commit, the first picking of the whole tree (Vendors to Input)
appeared as a direct child of the final picking,
as well as a child of the intermediary picking (as it should)

- Quality > Stock
	|
	- Input > Quality
	|	|
	|	-Vendors > Input
	- Vendors > Input

After this commit, the final picking contains only the intermediary
which in turn contains only the origin

- Quality > Stock
        |
        - Input > Quality
                |
                -Vendors > Input

OPW 1970286

closes odoo/odoo#32953

Signed-off-by: Lucas Perais (lpe) <lpe@odoo.com>