[FIX] purchase_stock: compare only date in on-time delivery

Previously, when compute on-time delivery, we compare the date_planned
on PO line with the date on stock.move. Since they are both datetime,
the product that delivered on the scheduled date may be considered delay
because of the 'time' part is late, which is normally not what we want.
In this commit, we only compare the date part of them to decide whether
it's late or not.

Task 2346121
PR #58653

Signed-off-by: Arnold Moyaux <amoyaux@users.noreply.github.com>