cubicweb #2465659 c-c db-check tells two opposite messages wrt is/is_instance_of relations [validation pending]

one say the relation will be added, another the entity will be deleted

prioritynormal
typebug
done in3.15.4
load0.100
load left0.000
closed by#6151b0199bc1 [c-c db-check] don't skip is/is_instance_of mandatory relation to avoid telling two opposite messages when the relation is missing. Closes #2465659